iMac RAM upgrade: Here's an affordable way to improve multitasking - CNET
HOW I UPGRADED MY iMAC 21.5in Late 2013 MODEL (SSD & RAM UPGRADE) - YouTube
Apple iMac 21.5" Late 2013 Core i5 8GB RAM 250gb SSD - VESA Mount Only NO STAND | eBay
Install memory in an iMac - Apple Support
21.5-inch iMac (Late 2013) Review: Iris Pro Driving an Accurate Display
Apple iMac 21.5-inch and 27-inch (Late 2013) reviews | Macworld
OWC SSD Upgrade Kits for 21.5-Inch iMac (2013 - 2019)
A-Tech 8GB RAM for Apple MacBook Pro (Mid 2012), iMac (Late 2012, Early/Late 2013, Late 2014, Mid 2015), Mac Mini (Late 2012) | DDR3 1600MHz PC3-12800 SODIMM 204-Pin Memory Upgrade at Amazon.com