first, make sure power is off, and comp is unplugged.
then you just need to find the screws to open the case. once the case is open, unplug the 2 cables attached to the harddrive and unscrew the screws holding the harddrive in place.
then again, the case my be opened by latches, scews, or a combination of both (you just need to find them). once the case is off, just follow what i said above about removing the hd.