How should I get better in Middlegames?

I am asking this question because I want to know how should I get better in Middlegames. 

One way is to solve tactics puzzles. Another way is to solve positional puzzles. Don't aim for something too hard .. like Imagination in Chess has some great puzzles but if you're unable to solve them you'll just end up frustrated. And thirdly, surprisingly, study the endgame.

thanks

Lots of possibilities:
Simple Attacking Plans by Fred Wilson (2012)
https://web.archive.org/web/20140708090402/http://www.chesscafe.com/text/review874.pdf
http://dev.jeremysilman.com/shop/pc/Simple-Attacking-Plans-77p3731.htm
Logical Chess: Move by Move by Irving Chernev (1957)
https://web.archive.org/web/20140708104437/http://www.chesscafe.com/text/logichess.pdf
The Most Instructive Games of Chess Ever Played by Irving Chernev (1965)
https://chessbookreviews.wordpress.com/tag/most-instructive-games-of-chess-ever-played/
Seirawan stuff:
http://seagaard.dk/review/eng/bo_beginner/ev_winning_chess.asp?KATID=BO&ID=BO-Beginner
http://www.nystar.com/tamarkin/review1.htm
50 Essential Chess Lessons by Steve Giddins
https://web.archive.org/web/20140708100833/http://www.chesscafe.com/text/review534.pdf
http://www.gambitbooks.com/pdfs/50_Essential_Chess_Lessons.pdf
Simple Chess by Michael Stean
https://web.archive.org/web/20140708104258/http://www.chesscafe.com/text/review400.pdf
http://store.doverpublications.com/0486424200.html
Amateur's Mind by Jeremy Silman
https://web.archive.org/web/20140708094419/http://www.chesscafe.com/text/ammind.pdf
https://www.silmanjamespress.com/shop/chess/amateurs-mind-the-2nd-edition/
Chess Secrets: The Giants of Chess Strategy by Neil McDonald
https://web.archive.org/web/20140708092313/http://www.chesscafe.com/text/review620.pdf
Chess Strategy for Club Players by Herman Grooten
https://web.archive.org/web/20140708101926/http://www.chesscafe.com/text/review696.pdf
https://www.newinchess.com/media/wysiwyg/product_pdf/9042.pdf
Understanding Chess Middlegames by GM John Nunn
https://web.archive.org/web/20140627012322/http://www.chesscafe.com/text/hansen154.pdf
http://www.gambitbooks.com/pdfs/Understanding_Chess_Middlegames.pdf
Attacking Chess for Club Players by Herman Grooten
https://www.newinchess.com/media/wysiwyg/product_pdf/9032.pdf

Chess for Hawks

https://www.newinchess.com/media/wysiwyg/product_pdf/9041.pdf

The Chess Attacker’s Handbook

https://www.newinchess.com/media/wysiwyg/product_pdf/7754.pdf

Chess Strategy: Move by Move by Adam Hunt

https://web.archive.org/web/20140708093249/http://www.chesscafe.com/text/review890.pdf

How to Reassess Your Chess (4th ed.) by Jeremy Silman
https://web.archive.org/web/20140708095832/http://www.chesscafe.com/text/review769.pdf
https://www.silmanjamespress.com/shop/chess/how-to-reassess-your-chess-4th-edition/
The Art of the Middlegame
http://dev.jeremysilman.com/shop/pc/Art-of-the-Middlegame-The-77p3554.htm
http://store.doverpublications.com/0486261549.html
Pawn Structure Chess by GM Andrew Soltis (2013)
https://web.archive.org/web/20140708101523/http://www.chesscafe.com/text/review908.pdf
The Power of Pawns by GM Jörg Hickl (2016)
https://chessbookreviews.wordpress.com/tag/the-power-of-pawns/
https://www.newinchess.com/media/wysiwyg/product_pdf/9034.pdf
Modern Chess Strategy by Pachman
http://store.doverpublications.com/0486202909.html
Zurich 1953 by Bronstein
http://exeterchessclub.org.uk/content/review-zurich-1953-bronstein
http://theweekinchess.com/john-watson-reviews/john-watson-book-review-106-zurich-1953-by-najdorf
http://store.doverpublications.com/0486238008.html

thank u but I need more info

Try playing through recent and historical master level tournament games. You won't understand everything, but you will start picking up useful information right away. Like, when and where to make pawn breaks in typical positions. Getting familiar with basic plans flowing from your favorite openings, etc. Then, once you're familiar with typical plans and structures, you might study an advanced strategy book, or two. Also, the strategy lessons here on chess.com are excellent. But playing through complete master level games is a real eye opener. Youtube has tons of top level games showcased by prominent authors and coaches. The only thing holding you back, is actually doing this stuff.

play

Just read My System by Aron Nimzowitsch. I found it very instructive.

Study positional chess. Such as where your pieces belong, maneuvering formulating a plan and creating and exploiting weaknesses 

One can get some idea of the lasting scope of the respect for My System by looking at:
https://www.chess.com/article/view/the-best-chess-books-ever
Still, it might be noted that My System apparently did not occur to GM Yasser Seirawan as something to include in his list of personal favorites, and Aaron Nimzowitsch was not identified by the GM as a very worthy author.

https://www.chess.com/blog/RoaringPawn/an-open-letter-to-the-four-time-us-chess-champion-gm-yasser-seirawan

https://www.chess.com/blog/GMYAZ/open-letter-response-user-radovics-letter-to-me

My System has accumulated some direct negative commentary over the years.
"... I found [the books of Aaron Nimzowitsch to be] very difficult to read or understand. ... [Nimzowitsch: A Reappraisal by Raymond Keene explains his] thinking and influence on the modern game in a far more lucid and accessible way. ... The books that are most highly thought of are not necessarily the most useful. Go with those that you find to be readable; ..." - GM Nigel Davies (2010)
In 2016, IM pfren wrote:
"My System is an iconoclastic book. A lot of things in there is sheer provocation, and it does need an expereienced player to know what exactly must be taken at its face value.
I love 'My System', and I have read it cover to cover one dozen times, but suggesting it to a class player is an entirely different matter."
Also: "[Some things] ARE wrong, and it's not easy for a non-advanced player to discover those wrong claims.
Nigel Short has claimed that 'My System' should be banned. Stratos Grivas says that the book is very bad. I don't share their opinion, but I am pretty sure that there are more useful reads for class players out there."
Although he is a fan of My System, IM John Watson similarly acknowledged (2013) that:
"... Not everything in it has stood the test of time, ..."
http://theweekinchess.com/john-watson-reviews/john-watson-book-review-108-of-eplus-books-part-2-nimzowitsch-classics
One last point to keep in mind is that, even if My System would eventually help a player, it might not necessarily be helpful to a player now.
"... Just because a book contains lots of information that you don’t know, it doesn’t necessarily mean that it will be extremely helpful in making you better at this point in your chess development. ..." - Dan Heisman (2001)
https://web.archive.org/web/20140626180930/http://www.chesscafe.com/text/heisman06.pdf
For those who have never seen it, a My System sample can be seen at:

https://www.qualitychess.co.uk/ebooks/MySystem-excerpt.pdf

A Chess Praxis sample can be seen at:

https://www.qualitychess.co.uk/ebooks/ChessPraxis-excerpt.pdf

Various samples:

https://www.newinchess.com/media/wysiwyg/product_pdf/9027.pdf

Okay...... but personally I find it very instructive and I do not play at that caliber.