change instrument

If you want to totally change the instrument on the staff, the right click the staff, choose staff properties and click the change instruments. If you want to temporarily change the instrument see https://musescore.org/en/handbook/mid-staff-instrument-changes

Also, if you're trying to write for something other than just piano and are starting with the default empty score then wanting to change it to something else, the better thing to do is simply create a new score already set up for the instrument(s) you want, with File / New. The default score is really just there to play around with, you should normally be starting new pieces with File / New.