You can't in a few days (other than not eating or drinking in which you will lose weight but not fat). Weight lose if pretty simple: you need to burn more calories in a day than you eat. Cut out (or at least reduce) high fat/high calorie foods and exercise more, to burn more. Diets don't really work unless you up your calorie burning: walking, use the stairs rather than lift, swim, dancing, gym......
As I understand we as a nation eat about the same number of calories as we always have over the last 1000 years. The reason weight gain is so prevalent is the lack of exercise. Find something you enjoy and burn those calories!