Most Linux drivers, be they NIC, hard drive controler, or any other hardware drivers, are typically either compiled into the actual Linux Kernel itself, or stand-alone "modules". To work with modules, you would want to read up on the following man pages (by typing "man command
As for updating NIC drivers; again, most "drivers" in Linux come with the kernel, so you have two avenues you can normally persue;
1) Beginner Linux Users:
Check your Linux distribution of choice, and see if they have any updates available. If there's a "Kernel Update", that's probably what you're looking for.
2) Advanced Linux Users:
Check and download the latest kernel from kernel.org and start recompiling
. Once that's done, install the new kernel and give it a test-boot.
A Word about Third-party NIC drivers:
Most of the time, you'll find what you're looking for in the Linux Kernel source tree (the unpacked tarball thingy you get off of kernel.org). However sometimes there's new drivers, or the actual hardware vendor has something on their website you can pull down and setup. Typically the only time you're going to come across something like this is for an eccentric wireless network device.