Before updating your FP2 to the latest release using the updater app, firstly go to Settings -> Display -> Sleep and set the time to 30 minutes. (Default 1 minute).
When the phone goes into sleep mode, the update seems ‘stuck’, setting the sleep time to 30 minutes overcomes this issue.
After the update has been done, set it back to the time you prefer.
Even better: Go to developer options (you’ll have them as soon as you tapped the built number 10 times - in settings > about phone) and tick “stay awake”. Then plug in your phone while updating.
With stay awake the phone will never sleep while plugged in.
No, it wasn’t. I am on 16.05 and it works. Really strange that it is greyed out @Friek. Did you install any app that is able to control your display settings?