You can reduce pressure by twisting the red knob on the over pressure valve, this will vent through a pipe in an outside wall. You should establish the cause of the overpressure.
Does it only happen when the boiler is hot, and drop right down when all of the heating system is cold?. Suspect expansion vessel in need of recharge. Common problem.
Otherwise....
Does it stay high with the boiler cold?. Suspect filling loop leak from water main (disconnect filling loop to see if it drips) /cures problem.
or .. (worst of all):
leak between the sides of the secondary heat exchanger (turn off the cold water feed, open a hot tap, does the pressure gradually fall, with a dripping hot tap?).
c, December 2007