Here are the necessary steps to update SQL Server:
- Obtain a valid installation media and license.
- Stop the "Hyper-V Time Synchronization Service" service.
- Change the system date on the server, to a date before the trial expiry date.
- Run SQL Server installation and choose "Upgrade from preview version"
- Start the "Hyper-V Time Synchronization Service" service again.
The reason for changing the system date is that the upgrade won't run if it can't start the existing SQL Server service.