Tonality lives with the key signature, so this is the same pair of buttons that transposes a song.

 

On iPhone, iPad and Android, open the Editor, tap Info (i), tap Key signature, select the key and Major, then tap Set and Transpose. On Mac and Windows, open the Editor, set the key signature at the bottom of the Editor panel, click to set Major, then click Set and Transpose.

 

For more, see Transpose a song on the iReal Pro website.