Use a Phillips driver to remove the eight screws securing the back cover:

Four 9.5 mm screws

Four 5.8 mm screws

Despite the standards, Phillips screwdrivers can vary in size and shape—two drivers labeled as the same size may fit differently in the same screw. Use the size that fits the snuggest into the screw head.

Throughout this repair, keep track of each screw and make sure it goes back exactly where it came from to avoid damaging your Steam Deck.