Standard RAM: 8 GB Maximum RAM: 16 GB*
RAM Type: PC3-12800 DDR3* Min. RAM Speed: 1600 MHz
Details: *By default, this system has 8 GB of 1600 MHz DDR3 SDRAM. Apple does not intend for one to upgrade the RAM in this system. However, as first reported by OWC, this system has two SO-DIMM slots (204-pin). Unfortunately, accessing these slots requires one to pry off the display that is glued to the case and remove the logic board.
Scroll down to get to the installation video on this link: http://everymac.com/systems/apple/imac/imac-aluminum-tapered-edge-faq/how-to-upgrade-imac-ram-aluminum-2012.html