There's a single screw in the back. Unscrew it. Then pull the tab-like things and lift up to open the case. Set the top aside. Unplug the power and SCSI cables to the HD (it's opposite the floppy drive. also note that it may be easier to do this after the next step). Pull the plastic hooks and remove the metal HD cage (this takes awhile if you try it alone - it's some stubborn plastic). Unscrew the HD from the cage, and screw the new one in. Reattach the SCSI and Power cables. Push the HD cage back into the holder and make sure the plastic hooks catch. Replace the top cover, and screw the screw back in.
If that all sounds complicated, don't worry. Once you get the top off and realize how to get the cage out, it becomes obvious. Oh, and remember that you have to use a SCSI drive, not an IDE/ATA.
