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)
and that is converted into machine code:
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!