It was surprisingly easy to upgrade, took less than 5 minutes of my time, saved me lots of money and increased the total memory over and above what Apple sells even at their max (32 GB).
Here are the steps:
1. Unplug your iMac and remove the door to the memory (located on the back of the computer). (I used a screwdriver to push the latch open.)
2. Remove the currently installed memory chips.
3. Replace them with the newly purchased OWC chips, , in this case 4×16 GB, being careful to push them gently into place.
4. Close the door, replug everything connected to your iMac and voila! (So easy, even a PrEditor like me can do it!)
5. Gloat about how much memory you have and watch your applications become screaming fast.