Before you take out the silver screws, use the metal loop above the blue screw to pull out the old PS3 hard drive. It should be pushed towards the front of the PS3 before pulling it out to disconnect it. Once it is out you can carefully remove the silver screws to detach the hard drive from the metal frame. There should be four in all, two on each end.
Your newPS3 hard drive upgrade should go where the old one once was. The size limit on replacement PS3 hard drive upgrades apparently tops out at 250GB, so use a larger hard drive at your own risk. Also, make sure that you get the right kind of hard drive (2.5" SATA Notebook HDD).
Slide the new hard drive, secured in the metal frame, into the PS3 and push it toward the rear to connect it to the system. You should now be able to replace the blue screw. Then put the cover back on and turn on your PS3.