you will find in the toolbar - category < Clef / Key >.The buttons you will find in the toolbar - category < Clef / Key >.
Click at the position, from which the new clef should be valid.
Afterwards, click on the corresponding button.
Clefs, which don't stand at the beginning of a line, are Symbols and can be handled like other symbols.
If you click on this button (Icon) 
a clef selection dialogue appears.
Clefs at the beginning of a line become through entitles deleted.