How can I reduce pressure in my Worcester 350 combi?

How can I reduce pressure in my Worcester 350 combi? My 13-year old boiler crashed and I used the manual tap to repressurise the system. No problem. Worked first time. Left the tap open slightly, whilst the system was running and now the pressure is too high and the overflow is running whenever the boiler is working. How can I depressurise the system to get it down from 3-4 bar to