Use a JIS 000 driver or an official iFixit PH 000 driver to remove the following screws securing the back panel:

Two 3.6 mm-long screws on the top of the device

Two 3.6 mm-long screws on the bottom of the device

To prevent these tight screws from stripping, apply firm downward force, work slowly and try a different screwdriver if the screws won't come out.