Is there a way to take someone else's piece and edit it?

The title sounds really cringy, and it kinda is, but hear me out.

I want to make a remix of a video game soundtrack's piece, and the easiest way to do so would be to take an already-transcribed version of the song and work with it from there. Since Musescore has the option to play the piece through the computer and hear what it sounds like without having to actually play it (especially useful for a complicated piece like this with nigh-on impossible chords) it would be most convenient if I could work with it directly in the software. Is that possible in any way?

If I click on a score's key signature in the browser, it says that I can "Transpose this score to any key, with the MuseScore app for mobile". I downloaded the app but the key signature isn't even visible there.