Take the outer case off the computer and the drive will be held in by 4 screws. Unscrew all 4 (usually small nuts with Philips screw seatings) and take the cable off the back of the drive (it may have 2 little plastic securing clips that need to be pushed to the side). The drive should pull out through the front. Reverse the process to put the new drive in taking care to ensure that the cable is correctly fitted as it can be a little fiddly. Reboot the computer and it will find the drive straight away.