For MuseScore 3 users, see Brackets
Select the bracket and press Del
Drag a bracket symbol from the bracket palette to an empty space in the first measure of a system.
Drag a bracket symbol from the bracket palette to an bracket in the score.
In version 0.9.4 or later you can adjust the horizontal placement of a bracket. Double click a bracket and press Shift+Left or Shift+Right to move it left or right.
Double click on a bracket to enter edit mode , In edit mode you can drag the height of a bracket to span arbitrary staves of a system.