MuseScore -- Index -- Clefs

Clefs are created or changed by dragging a clef symbol from the clef palette to a measure or another clef.
<F9> toggles the palettes window.

Change
Drag a clef from the palette onto a clef in the score. You can also drag a clef from the score to another clef of the score by using <Shift+leftMouseButton+Drag>.

Add
Drag a clef from the palette onto an empty part of a measure. This creates a clef at the beginning of the measure. If the measure is not the first measure in the staff it is drawn smaller.

Remove
Select a clef and press <Del>.

Note that changing a clef does not change the pitch of any note. Instead the notes are moved.