Set the Default key signature: 

click on Edit Song (pencil icon) and under Key signature, select the key & tonality (major/minor) you want and tap on Set.

If you select Set, only the Default key signature of the song will be changed – without changing any of the chords. See this post for troubleshooting: Key doesn't match

Transpose all chords in chart and change the Default key signature: 

select the desired key signature and press the Set and Transpose button.