HOW CAN I DEFROST MY FRIDGE FREEZER/ HAVE MISLAID INSTRUCTION BOOKLET?

My fridge/freezer is encased in a wooden "cupboard" and I cannot get at the power point. The freezer is quite iced up and not working efficiently so I need to defrost it. If I turn to "off" the internal switch which is in the fridge and leave both doors open, will both the upper fridge and the lower freezer defrost? . The appliance is Hygena model 6825 and is about 7 years old. I think that I must have left the door ajar for it to ice up. This has not happened before.