Q. How do I update a driver on Nano Server?

A. To update a driver on Nano Server the current driver should first be deleted and then install the new version:

pnputil /delete-driver driver.inf
pnputil /add-driver .\driver.inf /install

If you have a driver without an INF file it cannot be installed on Nano Server however in most instances there is an INF file inside the driver installation EXE which can often be extracted which id documented at https://technet.microsoft.com/en-us/library/dd759151(v=ws.11).aspx.