Under the laptop there should be an area usually under the palm rest area with about 2 or 3 screws. Undo the screws and pop the plastic off and you should see a HDD ( hard disk drive ) . MAKE SURE YOUR LAPTOP IS OFF when you do this
The HDD is usualy just plug and play, you should not have problems with this easy peazee.
Remove it and place it back into the slot making sure it fits snug, you can start the machine up.. make sure that the HDD does not fall out of the slot!! - ( at this stage you can hear / feel if the drive is alive or not )