You must select note events before you can edit them. You can select notes by clicking, Shift-clicking, dragging to make a marquee selection, and using the keyboard at the left edge of the Piano Roll Editor.
Click the note.
Note: If you select the macOS preference “All controls” for Full Keyboard Access in System Preferences > Keyboard > Shortcuts, you can use the Right and Left Arrow keys to select a note.
Shift-click the notes, or drag to enclose them.
The Piano Roll Editor header shows the number of selected notes, along with the region name. When multiple overlapping notes are selected, the Piano Roll Editor header shows the chord name. When no notes are selected, the header shows the name of the most recently selected MIDI region, or the number of regions selected.
Click the key on the keyboard at the left edge of the Piano Roll Editor.
To select all notes for a range of pitches, Shift-click or drag over a range of keys on the keyboard.
This is similar to selecting all regions on a track by clicking the track name.
Do one of the following:
To select the previous event: Choose Edit > Select > Previous Event.
To select the next event: Choose Edit > Select > Next Event.
Choose Edit > Select > All Following.
Click the track header in the Tracks area.