First put a couple of small pieces of tape to mark where the wipers sit when they are off - you probably want to put them back in the same place.

Remove the nut completely and put some penetrating oil around the threads. Gently tap the end of the arm in a couple of directions, Letting it sit for a while may help.

What is under there is a cylinder that has splines all around it, The wiper socket is splined also. If it has been on for a long time, the extra surface area can make it stick pretty good.

You will need to pull, wiggle, pry, and whatever you can come up with in order to pull the socket off the cylinder.

You do not want to damage the mechanism.
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '95 855, '95 854, the first three are mine, heh, heh, 485,000 miles put on 9 bricks