How do I loosen the sprinkler head adjustment screw frozen in place?

check the directions on the box. sometimes sprinkler head has two screws. one screw is for pattern adjustment and the other is for spray adjustment

The adjustments can be made using the instructions in this manual from Rainbird website.
http://www.rainbird.com/documents/diy/man_sprayheads.pdf

The sprinkler head unscrews just like a bulb. Dig a hole around the head to below the connection.
Unscrew the old one and screw in another one. You might need a nopple on the new head to match the connection. Set up the new head with the same setup an the one in the ground before you unscrew it as water will come out of the pipe and fill the hole. You want to get the new head set before debris gets into the pipe.

Turn the sprinkler on. Go to the sprinkler and at the center of the pop-up nozzle you should find a screw.If the screw is there, it can be tightened or loosened to adjust the spray distance.

Sounds like "Sure pop" was misleading. Try unscrewing the top or entire sprinkler then the top remove the spray nozzle to access the spring. Stretch it out longer and re-assemble the sprinkler screw it back in place with the spray notches on nozzle facing are you want covered. This should solve the problem if not, replace the pop-up.

Check to see if the small slotted screw is flush with the top of the spray nozzle. If so, it has been manually lowered. Turn the screw counter clockwise until it stops, be sure to hold the nozzle to keep it from turning. If no, unscrew the nozzle and remove debris from around filter. Tell the gardener to just mow the grass and not to touch the sprinkler system he is usually why it was turned down 9 times out of 10. If he says "No touch, No touch. He is lying fire him.

Try this. Should work.

www.mickman.com/irrigation/tips/toro600.html

I googled Hunter sprinkler heads and where to purchase them. In my area (the East bay, Tri-Valley) in Nothern California.

I went to Ewing Irrigation in Dublin, CA and found the Hunter sprinkler heads there. The man working the counter got two for me and he demonstrated how to change the head by unscrewing it from the base at or just below ground level by removing the old one and screwing in the new head. He also showed me how to adjust them with the spray pattern. You will need the little tool that comes with the sprayer head.

Making sure the center arrow is completely to the left and rotate the head to the right then back to left. They are set to a 180% spray pattern from the manufacturer. If you want a bigger pattern you turn the adjuster toward the ( + ) side and if you want less pattern you turn the adjuster to the ( - ) side. He showed me how to install the red sprayer inserts by simply pushing them in the cavity making sure the little u shaped indentation is pointing up to allow for the set screw to go through to adjust the spary.The spray varies in distance and amount of water being emitted from the head depending upon the insert used.

I was able to replace the Hunter head, adjust the spray and changed out the insert a few times until I found the one I liked. I used needle nose pliers to remove the initial spray insert that the man @ Ewing installed until I found the one that worked best.

I found using needle nose pliers makes it very easy to grasp the little inserts for replacing.
-Terry