If you raise the wiper arm so that the pressure against the windshield is eliminated, you can get a small terminal puller to pop the wiper arm off. A search for battery terminal puller on Amazon, eBay, etc. will give many purchase options. Or you can pick one up at a nearby discount parts store; also a nearby Harbor Freight.. It will probably be a little scary the first few times you do this. Wiper arm is secured to the shaft using a tapered, spline fitment and the final torque really locks it down. Norton Search

Likewise when you re-install the wiper arms, get them positioned where you want. Then while applying a small amount of pressure to secure it to the shaft, raise the wiper arm to finish tightening. Again, try to keep the pressure caused by resting against the windshield from applying a bending-moment at the shaft. I've had to repeat the removal & install a couple of times before I got the wiper arm where I needed it to be located. You can mark the end of the shaft with black scripto & put a small mark with a white scripto on the wiper arm (at the edge of the mounting flange rim; won't be seen when you install the nut cover).

Later,