Finally, they are detachable & not part of the logic board anymore, but the question is if you can fit a better card, Apple often uses custom parts & you might only be able to replace it with like kind...
The only way for you to upgrade is to return it and bring it back to Apple in 14 days and get your full money back.
The only way to get a better video card is to order a 27 inch iMac with the custom graphics chip option.
I am not sure if you can order this in an Apple store or not. The iMac options are listed on their online ordering site.
The custom GPU is a 1 or 2 GB VRAM GPU. It is only available in the 27 inch screen models.
If it has really been a full two weeks, you need to return this iMac like tomorrow.
Apple won't do the video upgrade for you as this is a non-upgradeable part by Apple standards.
If you attempt to upgrade/replace this GPU yourself, you void your 1 or 3 year AppleCare warranty with no hope of ever getting any free warranty work ever done on this iMac if anything should go wrong.
If your time for a full refund has passed, you need to sell this iMac so you can purchase an iMac with better specs and options.
Other than RAM, iMacs are not user upgradeable and you need to order the right iMac and combination of options options at actual time of purchase.
I just re-looked at your posts
Both 6750M and 6770M cards have both the 512 MBs of VRAM.
What is the actual difference in these cards.
Why is one better over the other?
In this case both 21.5 inch models and the 27 inch models have the 6770M GPU.
What is there to be gained?
The 27 inch models are the iMacs with the 1 and 2 GB VRAM GPU chips.