Before starting you will need to make sure you have a small cross-head screwdriver. Also make sure that you do not have wet hands!
Power off the laptop and unplug it. Turn it over so that the base is facing upwards. Now look for a panel with a Hard Disc Drive symbol on it, the symbol will look something like this:
Remove the screws from this panel (making sure to put them somewhere safe so that you son't lose them). You should then see something like this:
Its them simply a matter of sliding the disc drive out from the contacts (in the photo above you would slide it to the left, if it doesn't seem to work one way try the other, it may need a firm push, but don't be too rough). One the drive has been slid away from the contacts, simply lift it out.
The drive will almost certainly be in a "cradle", a metal panel screwed to the hard disc which keeps it in place when the computer is assembled (see photo below).
You will need to remove the cradle, which will involve removing about 4 screws from the long edges. Do not lose the cradle or the screws. Also take note of the orientation of the cradle with respect to the hard disc, as to put the new one in, you will need to put the cradle on the new drive before putting the drive into the laptop.
Just follow the above instructions in reverse to put the new drive in. If you need further guidance, the tutorial below is quite good (its where I got the images from):