For MuseScore 3 users, see Key signatures
Key signatures are created or changed by dragging a key symbol from the key palette to a measure or existing key signature.
F9 toggles the palette window.
Drag a key from the palette onto a key in the score. You can also drag a key from the score to another key on the score by using Shift+leftMouseButton+Drag.
Drag a key from the palette onto an empty part of a measure. This creates a key at the beginning of the measure.
Select a key and press Del.