Have you taken the fan off?
Have you taken the 3 screws behind the fan out?
It's the fan blade I'm asking about.
You need to take that off.
The nut in the middle is a left hand thread.
Once you have the fan (blade) off, you'll have access to the screws. (if you haven't already got them out.
Once the fan and screws are removed, it will come out the rear.
Hope this helps
You hold the fan with your hand.
Then you turn the nut clockwise to slacken it.
I hate to harp on, but please, make sure you are turning the nut clockwise (it's left hand thread).