Pressure valve stuck open

I have a shut off valve that is my sprinkler line pressure release. (see pics) It is the insides are stuck in the open position. I can turn the knob to open and close but it will still allow water to pass through. If it is in the closed position the water pressure is not as high as if it is in the open position. What do I do?

shut off the water drain it down and change it. it looks to be a compression fitting.

I would swap out the valve for an identical one. As the previous poster stated it's a compression stop. It looks like a cheapo 1/4 turn type. I would install a Brasscraft 1/4 turn stop.

Thank you all for answering my very plumbing illiterate question.

valve

Now that we have your drain valve problem solved, what is that ball valve on the main line? If that is your irrigation system control valve throw it away and install the REQUIRED pressure backflow preventer, or reduced pressure principal backflow preventer, whichever your area requires, between the house system and the irrigation system.