If it's only about the playback sound, then use the mixer.
However if you wish to compose for a different instrument, then you'd better start of creating the score for that actual instrument. That way you'll get correct transposition and probably already a close matching sound by default.
See https://musescore.org/en/handbook/create-new-score for how to create scores and change their instrumentation afterwards.
Comments
If it's only about the playback sound, then use the mixer.
However if you wish to compose for a different instrument, then you'd better start of creating the score for that actual instrument. That way you'll get correct transposition and probably already a close matching sound by default.
See https://musescore.org/en/handbook/create-new-score for how to create scores and change their instrumentation afterwards.