How to Upgrade Memory in a 21.5-inch or 27-inch iMac (Mid 2010) - YouTube
iMac 21.5" Mid 2011 How To Change/Upgrade Your Ram For Better Performance 2014 - YouTube
Simmtec 4GB RAM for Apple MacBook Pro (Early/Late 2011), iMac (Mid 2010, Mid 2011, Late 2011), Mac Mini (Mid 2011) | DDR3 1333MHz PC3-10600 204-Pin SODIMM Memory Upgrade Module at Amazon.com
A-Tech 32GB (4x8GB) RAM for Apple iMac (Mid 2010 27 inch 4-Core, Mid 2011 21.5/27 inch) | DDR3 1333MHz PC3-10600 204-Pin SODIMM Memory Upgrade Kit at Amazon.com
Maximum memory for 2011 iMac - Ask Different
Timetec 32GB KIT(4x8GB) Compatible for Apple DDR3L 1600MHz PC3L-12800 CL11 for iMac (Mid 2011, Late 2012, Late 2013, Late 2014 Retina 5K, Mid 2015 Retina 5K) SODIMM Memory Module MAC RAM Upgrade at Amazon.com
HOW TO: Upgrade The RAM to 32GB on a Mid-2011 21.5inch iMac - YouTube