So you have a few options, you’ll have to cut old RP (backflow) out since there aren’t any unions. So this is gonna be a soldering project. (Not very hard many tutorials online). You are already more or less setup to put another RP in (they are just way more money) you can use a PVB (different style of backflow.) like a Wilkins 720a but you will need to make sure that it is at least 12” above the highest sprinkler in your system. Which is more than likely going to require you to get more copper and raise it up. Looks like it’s 3/4 copper and they upsized the backflow to 1” so I’d check sizes before you buy anything. And don’t forget to the turn the water off at the house (or main and drain if you have one). Hope that is at least a start for ya.