For our model (PDW8480J10SS), the upper spray arm (beneath the top rack) can be removed using a nut driver. For the lower spray arm, lift up gently and rotate it counter-clockwise; it will "unscrew" from the 'fine filter hub nut' (454).

To clean these, I take them to our stationary tub and use a hose to blast the water in/out of every hole. I find it helpful to use an old dental tool (hardware store bargain bin) to help coax out the food bits. The ends are usually the worst, and may require some persistence to rinse all the junk out of the arm.