How do i lower the pressure on my boiler?

i need to lower the pressure on my boiler when cold its starting at 2.5 raising up near 3 when on it should start out between 1-1.5. The black switch used to put the pressure in was left open not my fault so i closed it today. I was told to bleed the rads but theres no air just water coming out. I need to know is it ok for me to let the water out of one of the rads and what effect will this have on the rads performance?? Its a vokera mynute boiler. or is there another way i can do this??
Please help me!