How to Upgrade/Replace the Main Drive in a 21.5-inch iMac (Late 2012 - Late 2015) UPDATED - YouTube
Upgrading a 27-inch iMac 5K to 14 terabytes - Replacing the Fusion Drive with both a SSD and an harddisk
Amazon.com: LEVEN 1TB Mac Upgrade NVMe SSD PCIe Gen3x4 Compatible with Apple MacBook Air (2013-2017) & Pro (2013-2015), Mac Mini (2013) & Pro (2014), iMac (2013-2019) : Electronics
How to Take Apart the Apple iMac 5K 27" iMac Late 2015 - YouTube
How-To: Now's the right time to swap your old iMac's hard drive for a fast new SSD - 9to5Mac
2015 iMac Reviews: 'Best All-in-One' Desktop, But Lacks USB-C and Fusion Drive Isn't Standard - MacRumors