Apple iMac (21.5-inch and 27-inch, Mid 2011) - Manual (Page 40)

40 Chapter 3 Boost Your Memory
4 Raise the stand and use a Phillips #2 screwdriver to loosen the three captive screws on
the memory access door. Turn them counterclockwise.
Phillips
screwdriver
Raise
stand
Memory
access
door
®
5 Remove the memory access door and set it aside.