Where do I replace memory on imac

Here is a linked support article: https://support.apple.com/guide/imac/install-memory-apdd768f6349/mac

Essentially, the first place you need look is hidden inside where the power cord is. You'll need to completely remove the circular plug from the bezel. Underneath, you'll see a button, that you'll need to push in quite far in order to eject the hatch. Once the hatch is free, you can then go in and gently remove the existing cards.

If you're replacing the memory sticks, be sure that you have compatible cards, and that they are placed in every other slot, not consecutively right next to eachother. If you're adding memory, just fill in the two remaining open slots.