Confirming that it is possible to update firmware on both individual devices as well as clusters using FortiManager.
You can easily create a group (or use an existing one) and right click to use Firmware Update. This has been the case for several patches if memory serves.
I' m sorry you had difficulty locating this feature, I will check to ensure the documentation is clear that this is where you need to go to do the update.
Cheers!
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Sean Toomey, CISSP FCNSP
Consulting Security Engineer (CSE)
FORTINET— High Performance Network Security