The error message "Could not find valid CAD install", appears due to the following two reasons in a windows machine:
1. Absence of the NX installation of a particular version.
2. Non-availability/creation of NX registry under Unigraphics Solutions folder, due to custom installations. 

Fig 1: Update Model in STAR-CCM+

While executing the "Update Model" command, STAR-CCM+ starts looking for NX installation. The above error message appears If it is not found.

To address the present error, please check if the correct version of NX is installed; there is also a need to check for the existence of NX registry under the Unigraphics Solutions folder (as shown above).
Due to custom installations, it is possible that NX registry was not created in registry under Unigraphics Solutions folder. Sometimes, it could also have been manually moved to some other location.
If either of this was not found (NX folder not created or moved to some other location), please add NX folder to its corresponding paths.

For NX 12 and below, please check the registry keys at Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Unigraphics Solutions\NX\12.x

For Simcenter 3D and NX 1847 and above please check the registry keys at Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Siemens\

Note: Typically, we don’t need to restart the machine for this change to take effect.