Select the desired key signature and press the Set and Transpose button in order to transpose all the chords in the song chart and set the Default key signature of the song.
If you select Set, only the Default key signature of the song will be changed – without changing any of the chords.
[For any song in the editor, always make sure the default key is the key the chords are in as they are in the editor. See here.]
More info under Soft and Hard Transposition.