1. When note input is active, you can shorten or lengthen the note you have just input by typing Shift+Alt+left or right arrow.
  2. You can change the amount by which the note is shortened or lengthen by increasing or decreasing the value of the rhythmic grid. The key commands for this are Alt+[ and Alt+].
  3. You can also edit existing notes to make the shorter or longer by using the same method. And if Insert mode is active (press I) then subsequent notes will move by the same amount).