21.5 inch late 2013 iMac - RAM upgrade - Apple Community
iMac 2013 Model Ram Upgrade - YouTube
Install memory in an iMac - Apple Support
Memory Upgrades for iMac 21.5-Inch 2012 - Late 2013
iMac RAM upgrade: Here's an affordable way to improve multitasking - CNET
NEMIX RAM 16GB Kit (2x8GB Upgrade) DDR3-1600 SODIMM PC3-12800 for Apple MacBook Pro Mid 2012 / iMac Late 2012 / iMac 2013, Late 2014, Mid 2015, Mac Mini Late 2012 at Amazon.com