Make sure you are logged out first.

1. Hit Ctrl+Alt+F1 and login using your credentials.
2. kill your current X server session by typing sudo service lightdm stop or sudo lightdm stop
3. Enter runlevel 3 by typing sudo init 3
4. Do whatever you need to do
5. You might be required to reboot when the installation finishes. If not, run sudo service lightdm start or sudo start lightdm to start your X server again.