Before removing the screws from the drive caddy make a note as to which way the drive fits into the caddy. This way when you fit the replacement drive you'll know which way around the drive goes.

There are four screws, one at each corner. Remove these screws and separate the drive from the caddy.

The drive can now be replaced. To re-assemble, follow the instructions in reverse. The case and keyboard should clip in quite easily.

Finally, remember to reattach the touchpad and keyboard when re-assembling!