What do I need to update a late 2012, 27" iMac to SSD?
Not "any" SSD: SSDs come in a wide range of shapes and connectors. But any SATA 2.5" drive (in the same old form factor as Hard Drives) should work.

The tear-down instructions are found in iFixit. It is marked as "difficult". If it looks risky, you may want to just use an external SSD, connected by Thunderbolt, which should be sufficiently fast.