My 12 year old cousin is 8st 5lb and wants to get to 6st 5lb. what do you suggest she does in order to lose this weight?

Combination: Make a mental choice to be healthier. To lose weight, you must burn more calories than you eat. Eating the right calories including the proper amount of protein is very important. Exercise will burn additional calories. However, strength training with elastic bands or weights builds muscle, which burns more calories at rest and also keeps metabolism up. No easy or fast fix. Work hard good luck.