There is not a way to persey "force" an update. The client polls every 4 hours for updates depending on your client/individual client configurations. If you deployed any clients initially with the autoupdate feature flag turned off (autoupdate=OFF) than this could also be a reason. If this is the case then I suggest pushing out a new client version with the flag turned on, and in the future will be able to manage this and update from the UI as per your configurations.
check the nsuserconfig.json to see if autoupdate=true or false and also check the STAUpdate.txt file and this will give you some insight to see if its trying and failing.