how to change ON/OFF times on Ideal combi boiler HE24 Isar ?

hi, i jus had Ideal, Isar24 installed like 2 days ago, but i am fiding it hard to use, the engeneer set the ON /OFF times for me but i wana change them or some times i just want to turn the heaterz on diff than those times, i tried to use the guide book but cant get the hang of it it wont jus come on. the hotwater is runing fine n it turn the boiler on whn the hot water is needed but apart from that i m jus stuck wiv it. ?