Change Instrument

See https://musescore.org/en/handbook/staff-properties#change-instrument

Press 'I' to add the Insrument, then copy and paste;
Open the Mixer to change the sound;
Right cilck a measure-> Stave Properties->Change Instrument.

If by this you mean you already have a staff for instrument but you want it to be for a different instrument, right click the staff, Staff Properties, Change Instrument. If you mean part way through the piece you want the player to change instruments - say, starting on flute but switching to piccolo - use the Instrument element from the Text section of the Palettes.

For more info, see the Handbook under "Staff Properties" and/or "Mid-staff instrument changes".

Or if you want to change the instrument from a special point inside the staff see: https://musescore.org/en/handbook/mid-staff-instrument-changes#add-inst…