Instrument Sound Selection
Among the staff controls, the Instrument Sound Selection button lets you change the instrument sound of a staff. You can change the instrument sound even while the song is playing.
If the staff controls are not visible, and you do not wish to display them, then while holding down the 'S' key (for 'staff'), click the mouse somewhere in the staff.
Composer will display a pop-up menu such as illustrated here:
In the pop-up menu, choose the Instrument Sound command.
For details, see Changing the Instrument Sound of a Staff.