Once you do this you'll be so embarassed that it was hard to figure out...
If you pull the wiper blade away from the windshield, notice where the arm hinges - just past that hinge you'll see a little tab on the top side of the arm. Using a small screwdriver, pry that tab out. Once it's out the arm will stay up. Simply pull the arm off the truck.
The arm just has small 'teeth' that hold it in position, and it may take a few tries to get it back on in just the right spot where it'll rest where you want it.
...and don't feel silly - it took me a little while and a lot of vocabulary to get the damned things off the first time!