Advice for a good tool to analyze masters' games.


First of all, go to https://www.chessgames.com/ .
There you can find many master games. Search by name, opening or year, and find what you're looking for. After that there are few tools you could download that are free. I am using PGN chessbook to analyze my own games, so you can use it for master games as well.


Sure. Open the tool and some game in it. Click on the move you would like to change to something else. And then click here:
Until you create variations of your own, in the circled area you will see "No variations". So click on it and there you will be able to move pieces and create any variation you want.
I suggest that you try to think of your own variations as well not only those mentioned by masters. That way, you will either come up with some viable move, or more likely see why some of your ideas do not work in that game.
And one other thing. I hope you will not mind if I give you a few more tips on how to improve, if that is your wish and you have the time. I wrote a blog for weaker players such as ourselves. It helped me get a bit better, so you might find it useful:
https://www.chess.com/blog/nklristic/the-beginners-tale-first-steps-to-chess-improvement

What is for premium membership? PGN chessbook is completely free.
You can download it from here:
https://pgnchessbook.org/