You do this by increasing the pressure on the spring holdiing the relief valve shut, so it takes more pressure to lift the seat.
Next to the oil pick up area of the pump you will see a round disc type affair roughly 15mm in diametre, this is the cap holding the spring and valve in against the seat.
Tap this in 2-3mm is all that's needed, don't need any more than this, excess pressure is no good and uses power to produce it.
tabetha