Just pull hard. We just replaced our motor and had to take the fan blade off. Was nervous so I sort of worked it back and forth as I pulled. Came off the old motor and went back on the new one.
I think there are two different types of blades so be sure to take the fan blade in to the rv store so you only have to make one trip to get the right blade.
If you mean the little "whizzer" fan with the 6 inch plastic blade, it just pulls off the motor shaft, the only thing holding it on mine is/was friction. Note when you pull it off the motor shaft has a flat side when you put a new one on.