How do I kill the x-server?

1. Use ctrl+alt+F1 to switch to terminal,
2. login
3. run sudo service lightdm stop, lightdm and xserver should be stopped now (check with ctrl+alt+F7, which is your current xorg session, it should not show any desktop now)
4. do your things
5. run sudo service lightdm start to start lightdm and xorg again.

Good Luck!

I did a little more digging around and found that I could just use pgrep to find its PID and then kill it.

pgrep dm -l

sudo kill (insert PID here)

You will need the help of htop or top. Write down the PID number of the process you want to kill (in this case, Xorg). Once you have the PID, run the below command in a root shell (sudo -s):

kill -9 <PID>

For example:

kill -9 1234

You should then be able to install the NVidia drivers.