How to lose belly fat without going underweight
Your abs are built in the kitchen, not the gym. In other words, it's all about what you eat. As a general rule, a man's body fat has to be under 10% in order to show the desirable six-pack. A woman's body fat need to be under 12%. This doesn't mean you can't have a "flat" stomach with a higher body fat percentage, just don't expect to be the envy of beach onlookers.
If you want to get a flat stomach while maintaining a healthy weight, you're just going to have to roll up your sleeves and do the hard work of eating a balanced meal three times a day with healthy snacks in between. There are no shortcuts.
However, there are a few alternative ways to aid you in your quest to that beach bod! I'd suggest you look into intermittent fasting. If you combine that nutritional regiment with a cardio and resistance program, you can see a flat stomach (and perhaps some guns!) in as little as six weeks.