If you are still testing the Enterprise version of Windows 10 you know it comes with 90 days.
Once this expires you can still continue to use but to extend the trial license you can run the following command in PS Admin:
Restart the system
You should now have a further 90 days.
This can also be done on Server 2016 which will give you another 180 days.
If this does not work make sure you are connected to the internet for the running of the above command restart and try again if it does not work the first time.
This is for test purposes only and should not be done in Production environments.