How can I reduce my Baxi Boiler Pressure ?

It was 0.4, so we opened up the valve to put more water in, but it went up too high (to 2.5). Trying to beled radiators and close the value just sent it even higher to 3.0. I dont want to put the heating on for fear it'll explode, so any ideas on how I can get it back down to a normal level ? Thanks.