The System76 Driver comes installed on your system and is located in your menu at System > Administration > System76 Driver. Any updates to the System76 Driver will be detected by Update Manager.
If you have performed a fresh Ubuntu installation, you will need to install and run the System76 Driver. To install it, go to...
...and download the latest driver .deb package. The latest driver will always be the second link from the bottom. Save it to your Desktop.
Once it has downloaded, double-click the icon on your desktop and follow the prompts to install it. Once you are finished, it will appear at System > Administration > System76 Driver.
The first time you run the driver after a fresh installation, you will need to click the Restore System tab and the Restore button. This will modify your software sources file so that any future driver updates will be available via Update Manager.
After running the driver the first time, you no longer need to use its restore fucntionality. You can just use the Install Drivers tab and the Install button to install any necessary drivers.
If for some reason you get a GPG Key error when trying to update, open a terminal (Applications > Accessories > Terminal) and run this command to fix it.
wget -q http://planet76.com/repositories/system76_dev.pub
-O- | sudo apt-key add - && sudo apt-get update