Can my manager go into my files and change my availability without letting me they will do that.?

No, it's not illegal for your manager to do that. In fact, that's one of your manager's responsibilities. There is nothing you can do if it happens.

First I assume you mean the company file on you, not your files.  And if so, yes, there is no law against doing so.  Second in 49 states you are an at will worker which means your employer can fire you when ever they like for virtually any reason and no reason at all.  So it doesn't matter when  you said you were available to work because you either work when your employer says so, or they will find someone else.  Sorry.

Let me guess, you're a liberal Democrat?