Navigate to the handbook page for MuseScore 3: Clefs
Clefs are created or changed by dragging a clef symbol from the Clefs palette into a measure or onto another clef. Use F9 (Mac: ⌥+⌘+K) to show or hide the palette window.
Note: Some clefs are only available from the master palette.
Drag a clef from the palette into an empty part of a measure - this creates a clef at the beginning of the measure.
Drag a clef onto a particular note to create a mid-measure clef - if the measure is not the first in the staff, it is drawn smaller. An example is a piano score, in which the top staff starts with a G clef and switches immediately to an F clef, then after a note and a rest, back to a G clef.
Note: a clef dragged onto a note (and especially onto the first note), will not be affected by 'Courtesy clef' properties.
To see whether the change will apply to the note or the measure, check what changes color when you drag it.
You can also select the note, or measure first, and double-click the clef on the palette to make sure it affects the right one.
Select a clef and press Del.