iMac 2013 sees iFixit teardown: SSD slot free for upgrade - SlashGear
21.5" inch Late 2013 iMac A1418 Hard Drive SSD Upgrade Replacement Repair - YouTube
Original 128GB PCI-e SSD Replacement for MacBook Pro Late 2013 - Mid 2014, MacBook Air Mid 2013 - Early 2014, iMac Late 2013 - Mid 2015, Mac Mini Late 2014 (Bulk, No Retail Packaging) : Electronics