I'm not entirely sure if it's possible, but it should be via the Task Monitor:
- the upgrade button should have resulted in a task in FortiManager
- you might be able to interrupt or delete the task
- the pane can be accessed via System Settings > Task Monitor or, if a Task is running, by clicking the icon in the upper right corner.
- or, though a rather ugly workaround, change the managed IP in FortiManager Device Manager (so it thinks the FortiGate is now on a different IP than it actually is); essentially make the FortiGate unreachable on FortiManager until such time as you can try the upgrade again.
+++ Divide by Cucumber Error. Please Reinstall Universe and Reboot +++