Changing most games without access to the source code isn't advisable for a beginner (even experienced programmers would generally stay well clear of it). Some games have been designed with modding in mind, and those can be easily edited (within bounds). And some games are built in ways that make them easy to edit (like many games made in HTML/Javascript).
But flash, unity, and renpy (I think) are all compiled - the code has been crunched down for the computer and isn't human-readable any more. You have to try to decompile it first, which sounds like a great idea but, in my limited experience, you just end up with a hideous mess. Even Twine games (though not technically "compiled") are minified, so editing them is a nightmare.
Sorry. I know that's not what you wanted to hear. On the up-side, it gives you a great excuse to start making your own games inspired by those games you love