7. Remove the old drive
Depending on how much data you have, it could take an hour or two to move all the data to the external hard drive (it took me about an hour and 15 minutes). When it's finished, shut the system down, flip the machine over and find the hard drive. I just left the bottom panel off during the back up; if you haven't, remove it again.
Loosen the four screws that hold the hard drive in place and put the screws into a small bowl. Finally, slide the drive out by its plastic tab.
After that, carefully loosen the screws that attach the drive to its bracket and put the drive aside. (You might want to use it as a spare.)