If you install the Nomadesk update via the Nomadesk Dasboard -> Help -> About Nomadesk -> Check for updates you will get a message that Nomadesk needs to be closed first.
In some cases the update is unable to close the Nomadesk process (it will keep repeating to ask to close Nomadesk), which you can do manually via taskmanager:
- right click the Windows task bar and select task manager
- look for the Nomadesk process under Background processes, select it and end it
- go back to the installer, it will be able to install the update now