There are several methods how to release license:
1. There is a way to configure the license manager to automatically reclaim inactive licenses by creating the options file with specified TIMEOUT feature. Default TIMEOUT feature has a minimum license release time of 900 seconds, which means your license will not be checked back into the pool before 15 minutes of inactivity has elapsed. License Administration Guide with the instructions how to create an options file is available at https://docs.nomagic.com/display/NMDOC/FlexNet+license+server+installation+and+licensing
2. Restart the license server itself. First stop the server from the Administration section of the license server management interface. Use the default address () or your server name (http://<server name>:8090) to connect to licenses server interface. Please note that stopping server shut downs and all vendor daemons. Active users will be notified and reconnect to license server would be needed. Start license server manually. On Windows platforms, open the installation directory in Windows Explorer and then double-click the lmadmin.exe file. Or run the lmadmin command with your desired command-line arguments on Unix platforms.
3. In order to release a hung license to the pool of free licenses please use lmremove command-line argument. Note that lmadmin's default setting disables the operation of lmremove, to enable it start lmadmin with the - allowLicenseReclaim argument. License Administration Guide with the instructions how to combine command line arguments is available at https://docs.nomagic.com/display/NMDOC/FlexNet+license+server+installation+and+licensing