How to roll back a video driver from the command line?

I decided to try to install the proprietary nVidia drivers. It should have been the '390 drivers but I installed '340 instead (forgot which graphics card I had). Now, when I reboot, the login screen is something like 800x600 and when I try to log in, I momentarily get a desktop (long enough to see the bluetooth/volume/power/etc/ icons at the top left, but they also seem to be at a low resolution) and then I get dumped back into the login prompt. I can access a terminal via CTL-ALT-F2. So how do I go about reverting back to the nouveau drivers using just the command line?