Note: if you have not done so, please read the introduction to Upgrading a PS3 Hard Drive before performing these steps.
Once the cover plate has been removed you will see that there is a hard drive carriage. secured by one screw. Use a phillips screwdriver to remove this screw, doing so will allow the old hard drive to slide out from the unit, from there you will have direct access to the PS3's hard drive, and you can change it. Move on to the next step.