Cut the carbs, sweets and processed foods out your diet. More greens, foods high in protein and healthy fats. Drink tons of water and lots of cardio. Theres literally no scientific answer. You just have to trust the process.
For example, if you have a stair master at your gym, HIIT would look something like this. 13 speed for 1 min. 3 speed for 30 seconds. Do that for about 10-15 minutes and I guarantee you will see better results