Toughbook hard drives are a little different from most laptops because they are cased in a specially designed, shielded caddy. Still, it's not brain surgery. Just take care when handling the delicate internal components, follow the simple instructions, and you'll be fine.
In this video, Marisa shows you how to replace a hard drive. Just follow along with what she's doing and you'll be fine.
Remember to power down your laptop completely. That means shutting it down (not putting it to sleep or putting it in hibernation mode, but shutting it down), unplugging the AC adapter, and removing the battery pack. It's also a good idea to discharge any static electricity you might have built up in your body before you start.