Ram is a user-replaceable part, meaning it will not void your AppleCare. It's ridiculously simple to install ram. There should be documentation included with your computer that outlines the procedures to opening up your computer and installing ram.
there is, it's actually printed on the underside of the current generation imac's foot. just lay it down on the desk with the screen facing the desk surface (which you're already carefully placed a towel on, to avoid scratches) et voila.