Comment 94 for bug 1292830

@Dan,
The way I used to change Nvidia driver version is to purge existed one then install newer.

detail steps:
1. switch to another tty console by alt-ctl + F${what-ever-number-you-want}
2. systemctl stop lightdm
3. dpkg -l | grep ${your-current-nv-driver-number} | awk '{print $2}' | xargs sudo apt-get purge -y
4. sudo apt-get install nvidia-${the-version-you-want}

please make sure that you can install nvidia-${the-version-you-want} before purging old drivers, otherwise it might get boot failure in next boot.