MuseScore -- Index -- Brackets

Brackets pallet Remove
Select the bracket and press Del

Add
Drag a bracket symbol from the bracket palette to an empty space in the first measure of a system.

Change
Drag a bracket symbol from the bracket palette to an bracket in the score.

Edit
Double click on a bracket to enter edit mode , In edit mode you can drag the hight of a bracket to span arbitrary staves of a system.