Hey there! No problem, I'm still very active in the Ren'Py community and so long as I'm here I'll be replying to tool questions haha. The Return text in question will be inside whichever screen you're using - the examples go in the order the screens are declared in the code. So, the first one shown is screen color_picker for example, which has a textbutton with "Return". You can modify those screens however you want; they're just made to be examples.
If you're ever not sure what piece of code is showing a particular element on your screen, you can nearly always hover over it with your mouse and hit shift+i to get a lot of information on it. It'll show you styles and such that apply and has links to where that code is in your project.
Hope that helps!