you don't - that is forbidden by the EULA.
The only exeption would be changing the icon, and for that you need a special program.
What do you want to edit there anyway? the game engine is in the scripts, game data in the archive files - the exe only contains the interpreter and some basics, but not the game itself...
And if you're talking about the sample games you can download - if they include a project file, you can open that in the editor as with any of your own projects.
If they are encrypted or don't have a project file, then you can't open them.