What's an effective way for me to lose belly fat and gain muscle simultaneously?

Diet and exercise. Only treatment that works is diet and exercise. Eat less then you burn off. 3500 calories to the pound. Will reduce your risk of diabetes, high bp, heart disease, cholesterol, infertility, etc. Direct exercise towards your abdominal muscles to firm them up. Otherwise, consider liposuction, you can increase your other muscles at the same time by exercise, but it's difficult while losing weight, .