You have to lift them up slightly and put a small rod into the hole near the base in the arm to get them off. Lift the arm, look at that circular hole, and lift it into a position where you can see clearly through the hole. Now insert a small rod (I use either a coat hanger rod or a thin hole punch) inserted into the arm. Now pull up on the wiper arm assembly and it'll come off.
It can be a bit of a you-know-what trying to do it the first time around, but it WILL come off.