This is the trickiest part of the whole operation. It took me a solid half hour to get the logic board out. I’m sure it would be easier a second time, because there’s not many delicate components around the USB/Internet sockets, so you can be a little more assertive with it than I felt comfortable doing.
To clarify, the connector pulls out in the plane of the logic card, not perpendicular.
Wuh! A few thoughts on this upgrade. First, don’t ever embark on this without having the right tools in place before you start. Splash out on the iFixit kit, because this one is not for the faint-hearted. I’ve upgraded every Apple desktop and laptop I’ve ever owned, and this is far and away the most tedious. Second, budget at least 3 hours unless you’ve done this a few times before (unlikely). There are plenty of points where you will want to take your time - so many fragile connectors to work on. Remember, you have to tear down the whole iMac to get to the RAM. It’s a shockingly poor design. I was so happy when my iMac fired up first time - I only gave myself a 50% chance!