Apple iMac (21.5-inch and 27-inch, Mid 2011) - Manual (Page 39)

39 Chapter 3 Boost Your Memory
Installing Memory
Your iMac has four memory slots. Two are filled with at least a 2 GB memory module.
You can install additional memory in the free slots or replace the memory in the filled
slots with 2 GB or 4 GB memory modules, for up to 16 GB of memory.
You can purchase additional Apple memory from an Apple Authorized Reseller,
an Apple Retail Store, or the Apple Online Store at www.apple.com/store.
WARNING: Always shut down your iMac and remove the power cord before installing
memory. Do not attempt to install memory while your iMac is plugged in.
To install memory:
1 Turn off your iMac by choosing Apple () > Shut Down.
2 Disconnect all cables and the power cord from your iMac.
3 Place a soft, clean towel or cloth on your work surface. Hold the sides of your iMac and
lay it down so that the screen rests on the surface and the bottom faces you.