How do i replace the o ring on the H/W valve?

Hi, Baxi 105e boiler is whistling when heating is on, if i run hot water tap it stops, have read that it can be as simple as replacing a faulty washer on ther H/W valve, can i just shut valve off and undo connections and replace? or do i have to pay someone £50 to replace a 50p washer!!!!!

Thanks for replys.