I had similar problem recently, I used a tool easily available from eBay.
https://www.ebay.com.au/itm/15mm-Windscreen-Window-Wiper-Arm-Removal-Remover-Tool-Glass-Mechanics-Puller/401753745342?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
Simply remove rubber cap, undo nut until...