You can change the pitch and duration (length) of selected notes in the Score Editor, as well as their MIDI velocity. The Velocity value signifies how hard the key was struck when the note was recorded. It often corresponds to the volume of the note when played back, but can control other parameters as well.
Select one or more notes, then do one of the following:
Drag the notes up or down.
Use Option-Up Arrow and Option-Down Arrow to raise or lower the pitch in semitones.
Edit the Pitch value in the Event inspector.
Use the Event Transpose +1 or the Event Transpose –1 key command.
You can also use Option-Up Arrow and Option-Down Arrow to raise or lower the pitch of selected chord symbols.
Select one or more notes, then do one of the following:
Edit the Length parameter in the Event inspector (shown as bars, beats, and ticks).
Use one of the Nudge Region/Event Length key commands.
Drag the right edge of the duration bar for the note horizontally to shorten or lengthen the note.
When Show Advanced Tools is selected in the Advanced preferences pane, you can show duration bars for selected notes or for all notes by choosing the corresponding menu item in the Score Editor’s View > Duration Bars submenu.
Do one of the following:
Select a note, then press and hold Command while dragging up or down.
Select one or more notes, then edit the Velocity setting in the Event inspector.
Select one or more notes with the Velocity tool, then move the pointer up or down.
In Linear view, select one or more notes, then drag the Velocity slider in the Score Editor inspector left or right.
The Velocity value appears in a help tag as you edit it. You can also hear the change, if the MIDI Out button 
is activated.