iMac 27" 5K Late 2015 Upgrade 8Gb To 16Gb Ram in 4K - How to upgrade Ram of a iMac 5K - 2017 - YouTube
Timetec 16GB KIT(2x8GB) Compatible for Apple Late 2015 iMac (27-inch w/Retina 5K Display)
Adamanta 32GB (4x8GB) Apple Memory Upgrade Compatible with Late 2015 iMac 27" Retina 5K Display DDR3/DDR3L 1867Mhz PC3L-14900 SODIMM 2Rx8 CL13 1.35v RAM DRAM at Amazon.com
How to Upgrade | Install RAM in iMac 5K 27 inch mid 2015 from crucial - YouTube
iMac RAM upgrade: Here's an affordable way to improve multitasking - CNET
Install memory in an iMac - Apple Support
Install memory in an iMac - Apple Support
Adamanta 16GB (2x8GB) Apple Memory Upgrade DDR3L 1600MHz SODIMM Compatible for iMac (Mid 2011, Late 2012, Early/Late 2013, Late 2014, Mid 2015), MacBook Pro (Early/Late 2011, Mid 2012), Mac Mini at Amazon.com