GarageBand for Mac: Transpose regions in the Piano Roll Editor
You can change the pitch of—or transpose—notes in MIDI regions to a different key. This is especially useful when you want to reuse a melody, rhythm part, or other pattern of notes, at a different pitch.
Transpose selected MIDI regions
In the Tracks area, select the regions you want to correct.
Make sure the Region tab is selected in the Piano Roll Editor inspector.
Drag the Transpose slider left or right to transpose the regions in semitones.
Transposing regions can lead to unwanted results, such as when the sound of an instrument moves out of its natural range. If the sound is unsatisfactory after transposing, try transposing the region up or down by an octave and see if you like the sound better.
Transposing applies to the entire region. To transpose individual notes, drag them up or down in the Piano Roll Editor.