Hi . your boiler has an expansion vessel inside. This is a metal vessel with a balloon inside. Thus takes up the expanded water.... As the water is heated it expands. This expanded water is taken up in the expansion vessel. If they vessel goes flat or fails, there is nowhere for the expanded water to go so it over pressurises the boiler . then when the water cools the pressure goes back down . the vessel needs pumping back up with a pump( car tyre pump works) to 1.5 bar.
Your expansion vessel is on the right hand side under the cover. You will see the place to pump it up . if after recharging it still goes up then your system may be too big and you may need to fit an extra one external on the pipework.