The current iMac does not have a user-serviceable hard drive - RAM is the only user-serviceable part. It IS possible to change the HD yourself if you know what you're doing, but you need to buy the correct tools to remove the glass panel from the front - and you'll need to remove the LCD too. Not for the faint-hearted and it WILL void your warranty.
You'll find a take-apart guide at
iFixit