In this example am moving from FMG_VM64_V5.6.6 to V6.0.3 which is a direct upgrade path according to the release docs

Steps to upgrade firmware:

  1. System  Settings > Advanced > Advanced Settings, enable Offline Mode
  2. System Settings > Dashboard
  3. In the System Information widget, go to Firmware Version field and click the Upgrade Firmware icon.
  4. Prompted with Firmware Upload box, click browse to locate package with the (.0ut file) that you downloaded from Fortinet Support and click open
  5. Click OK, the firmware will do a self extract and installation then reboot
  6. When the login window displays, log into FortiManager

                                   Congrats, you upgraded it !!

Offline mode prevents FGM from connecting to FGT hence preventing unwanted changes from happening during the firmware upgrade. Therefore, disable offline mode when firmware upgrade is successful