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$ sudo reboot
or
$ sudo init 5
Since there are at least two distinct parts to the nvidia and nouveau software – user mode (X11 driver) and kernel mode (kernel driver), there may be circumstances where a reboot is required. There also may be circumstances where blacklisting the nouveau kernel module are required. There is some information about how to blacklist the nouveau kernel module on this page: https://novel.slac.stanford.edu/twiki/bin/view/SCCS/BlacklistNouveau
But beware that some information on that page (BlacklistNouveau) is outdated and no longer relevant. It is just linked here for the specific blacklisting steps in case those are needed.
Additional Information below:
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