If the firmware-upgrade-services / -license for your USG (FLEX) / ATP / ZyWall / VPN has expired, all that has to be done is to again refresh the service license via MyZyxel and then on the USG via Configuration > Licensing > Registration , and another year will be added. See below screenshots as reference:
Myzyxel > My Devices:
After refreshing Service License:
It is not in our interest to charge our customers for firmware upgrades!