You need to unscrew the center part with the two holes in it, by using the belt to keep the pulley part from rotating, and using a special spanner with two pins projecting to engage the two holes. Remember, though, that the center part is on a left hand thread. If you try to turn it the wrong way you'll just tighten it and make it difficult for yourself.
There is another thread referred to from the one you posted: follow that reference for some discussion on the special spanner and removing the central retaining nut:
https://www.outdoorking-forum.com.au/forum/u...Words=diplomat&Search=true#Post17291
