Don't Upgrade The RAM In Your New Apple iMac, Do This Instead
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
Can you upgrade the RAM in the 27-inch iMac? | iMore
Install memory in an iMac - Apple Support (HK)
Install memory in an iMac - Apple Support (HK)
iMac with Retina 5K Display 2014/2015 Memory Upgrades