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.
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.
If you need to move a bracket further left or right, then double-click the bracket to enter edit mode, and press Shift⇑+← or Shift⇑+→.