Use a small Phillips (PH1) screwdriver. Some screws will need to be accessed from the bottom side of the case. One screw is concealed with a tamper sticker; the screwdriver will easily pierce the sticker.

BT-DU3 upper case held by screws in corners
BT-DU3 upper case seal covers over screw in corner
BT-DU3 upper case screwdriver to remove or install in corner