how do you adjust pressure on worcester 28i rsf boiler ?

i have been told that the pressure is to low on the boiler . the instruction manual does not mention how the pressure can be increased . states pressure should be between 1 and 1.5 bar, max 2.5 operating pressure. there is a black tap below the boiler , i assume that this is the inlet tap . but the instruction's do not mention the tap . would be greatful for any help