It should already if you told MuseScore which instruments you wanted when you created the score. That is, in the dialog that appears when you do File / New, either choose a template that includes the instrument you want, or select them from the list of instruments by clicking where it says "choose instruments"..
if you've already created a score for the wrong instruments, right-click the staff, then go to Staff/Part Properties, then Change Instrument to select the correct instrument.
The Mixer can also be used to change between sounds for a single instrument, like the varoipous different guitar sounds, or the different organ sounds. But it shouldn't be used to turn a piano into a clarinet, for example - the sound will be right but nothing else (transposing, range, clef, etc) will be.
Comments
Via Mixer (but you might want a dedicated soundfont).
https://musescore.org/en/handbook/3/mixer
It should already if you told MuseScore which instruments you wanted when you created the score. That is, in the dialog that appears when you do File / New, either choose a template that includes the instrument you want, or select them from the list of instruments by clicking where it says "choose instruments"..
if you've already created a score for the wrong instruments, right-click the staff, then go to Staff/Part Properties, then Change Instrument to select the correct instrument.
The Mixer can also be used to change between sounds for a single instrument, like the varoipous different guitar sounds, or the different organ sounds. But it shouldn't be used to turn a piano into a clarinet, for example - the sound will be right but nothing else (transposing, range, clef, etc) will be.