How to open & edit apk file in android studio

You can't, or at least not in a way that would be of any use to you. An APK file contains compiled code, not source - and that is a binary data that isn't human readable.
You write code in characters:

if (userCount > maxUsers)
   {
   RejectRegistration(ErrCode_TooManyUsers);
   return;
   }

and that is converted into machine code:

01A52C336E00005E5E23

Which does not contain info to "undo" it back to anything you would find useful!

To learn to code, get a book, or better a course. There is no short cut here!