You mean physically open the case?
Why do you want to? On the latest iMac models I understand that there's no access to the interior without removing the screen.
There's nothing in there that's user serviceable anyway, with the possible exception of the HDD (and it wouldn't be wise to play with that while it's in Warranty).
RAM is accessed via a plate on the bottom for iMacs.
If all you want to do is access the RAM as Noondaywitch suggests, instructions are here:
Otherwise, leave it alone. You cannot open it because Apple does not want you to open it. You need your warranty support--don't make it void.