Is it possible that you have a P35 intended to be sold in Spain? I've heard they have a 18 ft lb limit for Energy (about 24 Joules). My 5.5 caliber P35 came set for 30-35 fpe, about 44 joules. I don't know how they detune it but my guess is it is the regulator. It is adjustable on mine but you have to remove the pressure gauge and the nut it installs in from the end of the airtube. With that removed, you loosen a lock nut at the far end of the airtube, I think it is 12mm, and then turn the slot headed screw. You would want to go counter clockwise to raise the regulator setting. I would not turn it more than half a turn before retesting. You will need to replace the O-rings on the big nut when you put it back together. It is also challenging to get the big nut started, the threads are very fine. The hammer spring may also need to be turned up to get all the air out of the regulated chamber. I increase it until the velocity stops rising and then try small, like 1/4 turn, changes to see if effects accuracy. On some guns I've found a little more hammer spring helped. Some people recommend a little (5%) less for greater efficiency. On my P35-25 that caused the regulator to act up.
If you are in Spain or some other countries you shouldn't increase the energy if it will cause the gun to exceed the legal limit. In the U. S. there is no energy limit for airguns.