How to Upgrade the iMac 27-inch Retina 5K (2014-2015)
OWC 16GB (2x8GB) ram Upgrade PC3-12800 DDR3L 1600MHz sodimm 204 Pin CL11 Memory Upgrade Kit Compatible with Late iMac, Mac Mini, and MacBook Pro Computer at Amazon.com
How to Upgrade/Install Memory in a 21.5-inch iMac (Late 2012– Mid 2014) iMac14,1 iMac14,3 - YouTube
Install memory in an iMac - Apple Support
CMS 32GB (4X8GB) DDR3 12800 1600MHz Non ECC SODIMM Memory Ram Upgrade Compatible with Apple® iMac Retina 5K 27-Inch (Late 2014) Core I5/I7 - A6 at Amazon.com
Apple 27" iMac with Retina 5K Display (Late 2014) MF886LL/A B&H
Memory in New $1099 iMac is Soldered and Not Upgradable - MacRumors