Those old brass nozzles are solid & will last dang near forever, you probably have debris in the nozzle or maybe some calcification that is clogging it. No need to replace the nozzle, remove them & turn on the zone to flush as akaggie05 said. Clean the nozzle with some CLR and it'll work like new.

If you feel like you have to replace, expose everything and see how it is connected. Probably a 1/2" threaded tee down there. You can remove the riser & screw in a new schedule 80 shrub riser directly. Or better use a couple of marlex fittings to make a small swing-joint so the riser has flexibility & doesn't break off if something hits it.

As a irrigator of 19yrs I always try to leave the copper shrub risers. Those nozzles put out a lot of water compared to current day plastic nozzles. As the others have said, clean the nozzle w/ clr or similar and then put it back. If you need to relocate the riser just dig out around the riser and unscrew it and then screw in a 1/2" thread x barbed elbow and then connect that to funny pipe and then install another thread x barbed elbow with a threaded male adapter (gray) and then screw the riser into that.

Just wanted to follow up and complete the circle.

Dug down and unscrewed the risers from the pipe on three heads. Took them in the house and noticed that the spray head unscrewed from the risers so I took them off as well. Got out a pipe cleaner from the kids craft cabinet and shoved it down the hole in each nozzle. Sure enough in each head there was a piece of debris. Looked like a little piece of plastic. Maybe the size of a grain of rice. I fished that out, soaked them in CLR for a couple of hours just to make sure they were clean and reinstalled them. Turned on the system and they work perfectly. Nice spray of water with lots of pressure and coverage.

Thanks Texags! I really appreciate the help.