You suppose to secure the center bolt with a 5 or 5.5 mm hex key while turning the nut with a wrench.
My ususal trick as follows (assuming you haven't cause jamming of the nut due to accumulated debris on the threads and/or slight seizure)...
(a) get a small wire brush, brush off all the rust and debris off the tread as thoroughly as possible.
(b) soak where the nut meets the thread with PB blaster or kroil, repeat this process a few times during a few hours of waiting.
(c) carefully, undo the nut and if you feel slight resistance, back off (reverse wrench direction) and then spray some lube...
With care, you should, 7times out of 10, be able to undo the nut w/ ease. about 3 out of time due to some sort of rust or seizure, drastic measures will have to be sort (like lock plier + hack saw, etc.)
Q.