If you want to change an existing clef, the following procedure is a good way to do it.
Clef Change Procedure:
1. Select the clef, press Ctrl+E, and change it as necessary
2. Now, press Home, then Shift+End, which selects the whole staff (you can also try this procedure on smaller selections, where appropriate)
3. Press Shift+Ctrl+Up/Down arrow until the notes are at the proper position for the new clef
4. Press F9 to update the key signatures (this performs an audit of the internal score data)
You now have completed the clef change.