how can i increase water preasure in baxi bahama 100 boiler?

i just moved in a house and they have a baxi bahama 100 combination boiler and the water preasure is in the red. when i turn on the water it get hot for a few seconds then the boiler registers a fault and no more hot water. i keep resetting it but the problem seems to be the preasure but i do not know how to get the pressure up.