my freezers are iced up

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i have a fridge freezer and a chest freezer, well three days ago they were fine, now i went to get some chicken out just now and they are completely iced up, the fridge freezer is really bad and i cant get the ice out, so i have just left the door to it open. the chest freezer i have managed to bash all the ice off with a knife. also the chest freezer has a super button, does this need to be on or off? it was off before. and there is a nozzle with the numbers 1-6 saying colder. i had it on 3 but have now turned it down to 2. should this be higher or lower please? how can i stop this from happening again? and why does this happen, none of the setting or anything has been changed. the fridge freezer settings is on medium and the fridge is fine. what are the best settings and also what is the right way to defrost a fridge? i dont know as i have never had to do it before lol. thanks in advance for any responses.