Cut 3 pieces of wood a bit thicker than the new pads.
Jam then in the brake to relieve the pressure on the pads.
Undo 1 or 2 screws holding pad and pull out.
Reassemble reverse order
I work on commercial units ,and this sounds like a trunnion bearing, or drum has come loose
1 loose trunnion bolt -easy fix
2 bearings - call experience repair