The calculation of body fat percentage are based on three fat measurements: Triceps, abdomen and thigh. It is based on the Jackson-Pollock formula, so you would need a fat caliper to do the right measurements.
To reduce your fat percentage it’s recommended that you consult with experts in diet and exercise. Changes in eating habits and activity level is the way to go for the reduction of body fat.
You can also seek out to a gym that measures your body fat. They can give you a full body analysis using sophisticated machines and measurements.
See also how to calculate men’s body fat percentage and BMI.