iMac 27" late 2012 SSD upgrade
I have an iMac 27 Late 2012 and I upgraded my SSD.

The fusion drive is a misnomer - it's actually two drives.  There is an PCIe SSD, probably about 128GB that attached to the logic board, then there's a 3.5" 1TB HDD.  Technically, you could upgrade the PCIe blade SSD drive, but after some research, I found it wasn't really economical.

All you need is a standard SSD to replace that HDD. You then create a fusion drive using CoreStorage and you will end up with an SSD with 1.13 TB of total storage.

As per Everymac.com....





The basic steps for this upgrade are:


Back up data
Remove the screen
Replace the SSD (you will need a 3.5" to 2.5" HDD adapter
Replace the screen
Reinstall / Restore from Backup.


I've actually answered this question on another thread:  27 imac Late 2012 HDD Failure

While you are doing all these upgrades, I would upgrade the memory as well.  I used Crucial Memory and your iMac can get up to 32GB of RAM