How to adjust a pressure regulator, that doesn't have a screw/bolt?
My guess is that the screw is missing and you're getting the lowest possible pressure as a result. If it were me, I'd just replace it if you're able to access the other side and clear away the water under it (why is there water under it?). With access to both sides, replacing it is an easy DIY job if you're comfortable cutting and soldering pipes.