Clefs are created, or changed by dragging a clef symbol from the clef palette to a measure or another clef. Use F9 (Mac: ⌥+⌘+K) to show or hide the palette sidebar.
Drag a clef from the palette onto an empty part of a measure. This creates a clef at the beginning of the measure. Drag a clef to a particular note to create a mid-measure clef. If the measure is not the first measure in the staff, it is drawn smaller.
Select a clef and press Del.
Note: Changing a clef does not change the pitch of any note. Instead, the notes move to preserve pitch.