I was wondering if there was some sort of way to edit the code of quest files that I've downloaded from others?
I tried changing the file extension to aslx, but it gives me this error:

Failed to load game due to the following errors:
* Invalid XML: Data at the root level is invalid. Line 1, position 1.

So, I don't know what else to try. Is this just not possible?

I tried changing the file extension to aslx, but it gives me this error:

Try changing the extension to .zip instead. You should then have an archive which contains all the images and sound files, plus a single file called game.aslx.

However, this file contains both the game's original aslx file, and all the libraries it used - including Core or GamebookCore. It doesn't, however, include the core editor libraries, which tell the editor which tabs to display. This means that the editor won't be able to open this file, unless you add the library CoreEditor.aslx.

If you're comfortable with code, it will probably be easier to open game.aslx in a text editor.