It's really easy to change pitch or the key of a song. Click on the song and select "Edit BPM/Pitch Settings". Check "Change Pitch" and adjust the pitch by +/- 12 semitones. It generally sounds better to go down in pitch.
To adjust the pitch of all songs, click on the "Next >>" track and repeat the process. When you are all done, click "OK"
More on changing a song"s key