Best to always purge old driver before installing new one. Typically new driver does not uninstall old, may only overwrite part of it & then you get conflicts.
Always install nVidia drivers from Ubuntu repository. The nVidia .run file directly from nVidia is not updated to work easily with Ubuntu. You in effect have to reinstall with every kernel update with dkms. Repository has latest nVidia driver so no need to directly download it, anyway. And Ubuntu repository version is automatically updated into every new kernel.
You can see what is installed with these:
dpkg -l | grep -i nvidia
dkms status
lsmod | grep nvidia
This should show available versions, but best to install recommended.
ubuntu-drivers devices
If you just want default version - recommended one
sudo ubuntu-drivers autoinstall
Or you can manually choose any in list.
sudo apt-get install nvidia-XXX
If you have installed a previous version you must purge. And if installed older bumblebee, purge that also as not used now.
sudo apt-get remove --purge nvidia-*
sudo apt-get purge nvidia* bumblebee primus bbswitch-dkms
Installing Ubuntu-mate with dual boot option on Windows 10 - usb booting not happening