How do I increase the pressure on my Bosch condensing boiler?

I have a Worcester Bosch Greenstar 24i junior condensing boiler. It seems to be functioning fine - hot water and heating. HOWEVER it seems to be losing pressure - it is consistently in the 'red zone'. We were advised when it had it's first year's service that there may be a small leak under the floorboards and we will periodically need to increase the pressure in the system.
We are unsure HOW to do this. We have read the previous reply but are unsure if this applies to our model/system? Do we switch both levers/taps on at the same time to increase the pressure?

Any help in the simplest / non-technical language possible would be hugely appreciated.