There should be a screw on the fitting on the back of the air tube that is the bleed screw. On my first P35 is it a straight blade screw. On my later P35s and a couple bullsharks I have worked on it is a hex screw. But look on the rear of the air tube, the end closest to the action for a screw and turn it counter clockwise very slowly until you hear hissing. Back it off a little, let the hissing stop, then you'll probably have to open it a little more. Dumping the air rapidly will almost certainly damage the one way valve which you can fix with an O-ring but it's better to just let it out slowly.
Absolutely never open the airtube without letting the air out. You can pull a barrel or a hammer or the hammer spring with the air in but you cannot change the regulator without letting all the air out.
I would not buy a new regulator until you do a shot string, however, like I suggested earlier. Take some pellets you do not care for and shoot from full or nearly full down to something like 100 bar. If your regulator is working your velocity will stay with a fairly narrow range. Sometimes 10 fps but never more than 30 fps with a functioning regulator. If it starts low, then goes up as pressure decreases and then starts back down, your regulator has failed. This takes a little time and I would write down the velocity of each shot so you can look at it later but it will prove beyond doubt the regulator is your issue. Or not. I would not spend $100 for a new regulator without knowing the one I have is faulty.
But when you decide to replace it, after draining the air, you pull the nut off the end of the airtube where the degass screw is. You will have to loosen the clamping yokes which clamp the barrel to the air tube and then you unscrew the air tube from the action (it has to be empty or it will be a projectile). As long as you drain the air this is totally safe. Your gauge will tell you if the air is drained. If you are lucky, the regulator will come off with the nut. I you are not lucky it will be up in the airtube a little bit and you will need to either give the airtube a blip of air to push the regulator out or find a screw that fits the threads in the end of the regulator to pull it out. It is only the O-rings on the outside of the regulator holding it in. If you use air, be very careful and make sure the airtube is pointed at something safe. Very little air, a very quick open and close, should move it. Or you can take off the nut on the other end of the air tube and use something long to push it out. This is safer.
You seem to want maximum power which is tricky with these guns. The problem is the hammer spring is not real powerful and at higher regulator settings the hammer will not open the valve with the maximum force it can produce. That could be all that is wrong with your gun. If you maximize the regulator with the gun at 70 degrees F and then the ambient air temperature goes to 90 F your velocity can drop because the regulator is too high for the hammer spring at the higher temperature. The air at higher temperature has more energy. How high to set the regulator is thus a bit of a tricky question especially when we have no gauge so we have to use shooting results to see where the velocity falls to guess where the regulator is set. But enough (probably more than enough intro), I would set the regulator definitely below 150 bar and probably no higher than 140 bar. You could try 145 but recognize you may have to open the gun again and turn it down. 130 bar would be a safe choice but you might not like the velocity (although it will probably be higher than you are getting). Huma regulators have a paper tape around the regulator you can use to set it. I don't know how accurate it is but it's nice to have something. The SPA regulator has nothing.
So please do your shot string and find out if the regulator is working. If not, a Huma will be a simple fix. They are good regulators and Huma sells rebuild kits so they should last forever. If the regulator is working, I would turn it down (clockwise) half a turn. That can usually be done by taking the nut the gauge is mounted to off and using an 18 inch extension on a 12mm socket to loosen the lock nut and then a really long straight blade screwdriver to turn the screw. Tighten the lock nut, put the nut back on the air tube and fill with air. If turning the regulator down does not fix the velocity I would take the hammer out and look at it and the tube where it slides. I have never done this on a P15, only side cocking SPAs so I would look for a tear down you tube if you need to disassemble this area. There will be a pin from the cocking lever that may block hammer removal as well as the sear for the trigger. If you get them out of the way the hammer will fall out. You do not want to see sticky grease or oil on the hammer or the tube it slides in. That will reduce hammer force. Lube it with graphite if you want to lube it.
