I don't have an Flexmaster to do this however looked at FM docs for you. Here is the process. Please check and confirm to make sure that this is the correct steps. I used FM 9.5 user guide and it may vary little bit depending on your version so keep that in mind too.
Releasing Device Licenses. Devices can be released on a per-serial number basis:
1. Go to Configure > Common > Device Registration.
2. Click Registration Status.
3. Find the required Device Name in the table. Note the number of licenses consumed
in the Licenses column.
4. Click Delete in the Actions column and respond to the confirmation prompt.
FlexMaster deletes the device, releases the consumed license or licenses, and returns
you to the Registration Status page.
5. In the Registration Status page, verify that the device entry is deleted