RAM upgrade

What iMac?

If you bought a 21.5-inch model, the RAM is not practically upgradeable. The case is glued together.

It can be theoretically done but Apple will not do it. Some Apple Authorized Service Providers will not tackle that difficult job either. It requires almost a complete gutting of the computer to access the slots, which are on the "wrong" side fo the logic board; even the logic board has to come out. IT is NOT an end-user job unless you have expereince working moredn computer and tiny elctrical connections.

27-inch iMacs have user-upgradeable RAM.

Please see this Apple article on iMac RAM before spending any money:

Install memory in an iMac - Apple Support

noting this part:

Yes. But you can do it yourself. See Install memory in an iMac. You can also purchase RAM at a lower cost at OWC, Crucial, or DataMem.

If you bought a 21.5-inch model, and don't have enough RAM then consider returning it for a unit with sufficient RAM during the allowed return period. Also, make sure you get one with an SSD drive. It's much faster.