change my skill level

For what? Live games? Beat some players and as your rating increases, the opponents will get tougher.

You need to complete the David and Goliath quest by defeating the Ogre King in his Cave of No Return. It's not a particularly difficult quest if you prepare well.

When you created your account, the skill level choice is just a way to set your initial rating. It has no impact beyond that. If your strength is actually higher then the rating system will quickly get you to the appropriate place.

I said intermediate, but I suck, so I wanna go back to beginnner

Just play a few games. Your rating will quickly adjust.

want to be beginner

You were set up with an initial rating of 800. That is the beginner's level.

i went intermediate but am not at that level yet. how can i go to beginner

You can't change that after an account is created. However, the way Glicko works, the rating will quickly move to where it should be.

So do I accidentally picked expert and now I don’t play online

How can I go back on beginner?

Don't worry about it.  Your mistake was winning your first game against a 1200 player.  You mated him in 12 moves.

Now everyone thinks you know something about chess...

How can I go back on beginner? i dont want to be in my present move

I just joined up, my first game I got beat very easily.. am I automatically set up as a beginner? 
I hope so!

You pick but play some games as mentioned above and your rating will adjust and you will play people your level. Thanks 

I doubt your choice is even stored anywhere on the chess.com servers, it likely only determines your starting rating

Great, thanks. 

I started on intermediate but I not that good

Just play a few games and you will quickly get to where your strength in the pool is.