You'll need to manually release it. Release the pressure in the keg. Now that you've released the pressure in the head space, CO2 will come out of solution to create equilibrium between the beer and the head space again. Once it does, you release the CO2 pressure from the head space again, and so on until you get the carbonation level where you want it. Bringing the keg into warmer temps will help the gas come out of solution quicker.