how do I increase the pressure on worcester bosch highflow 400 combi boiler?

I have a worcester bosch highflow 400 combi boiler, about 14 years old. The hot water cuts in and out when the central heating is put on. I had the same problem last year and the engineer showed me how to increase the pressure, as that was the problem last year, I think it should be on 2, it is currently on 1. I do not remember how to do that (being a bit stupid!), can anyone tell me what to do?