Uninstall a nvidia driver completely

Commands can be executed to terminal. You can open a terminal with Ctrl + Alt+T keys combo.

If you remove --purge the nvidia driver you will be OK. No need to blacklist something, but sometimes maybe a force-load of the nouveau module needed.

First uninstall completely the driver.

Search what packages from nvidia you have installed.

dpkg -l | grep -i nvidia

except the package nvidia-common all other packages should be purged.

If you want to be sure that you will purge everything related to nvidia you can give this command

sudo apt-get remove --purge nvidia-*

the asterisk in the end means (Purge everything that begins with the name nvidia-)

BUT

above command will also remove the nvidia-common package and the nvidia-common package has as a dependency the ubuntu-desktop package.

So after above command you should also give the installation command for ubuntu-desktop package

sudo apt-get install ubuntu-desktop

Also sometimes the nouveau driver get blacklisted from nvidia driver. With purge command it should UN-blacklisted. If you want to be sure that nouveau will be load in boot, you can force-load it by add it to /etc/modules

echo 'nouveau' | sudo tee -a /etc/modules

Last , search for the xorg.conf file and remove it as well

sudo rm /etc/X11/xorg.conf

In summary

sudo apt-get remove --purge nvidia-*
sudo apt-get install ubuntu-desktop
sudo rm /etc/X11/xorg.conf
echo 'nouveau' | sudo tee -a /etc/modules

Although all above commands not needed, this is my way to completely purge the nvidia driver and use the open source nounveau.

More Info.