from their FAQ: Is there an easy way to change the key of a song?

1.Use Tools: Force Accidentals to assign an accidental to each note. 2.Select the key signature, and change it using Alt+Enter. If no key exists, then use Insert: Key Signature (or K) to enter a key signature at the beginning of the staff. 3.Select Tools: Audit Accidentals to update notes to the key. 4.If necessary to change the enharmonic spelling of notes, select Tools: Transpose Staff and specify an interval of "0".