Re: advice please. I am underweight but I still have so much fat flab?
If you are currently underweight or close to it, but still have excess flab, you may have too little muscle. Which means that your body fat percentage may be fairly high despite your low weight. That is generally considered unhealthy. However, it is also unhealthy to continue losing any weight if you are currently underweight or close to it.
It is likely that the best route to go near term is to gain muscle weight. This requires exercise; resistance / weight / bodyweight training tends to be the best for building muscles. Be sure to include big muscles like your back (pullups, rows), chest (pushups, dips), and legs (squats), as well as a strong core (crunches and variants). Eating enough, including sufficient protein, is also needed to build muscle.
Note that more muscle will increase your metabolism, so it will be easier to lose any remaining excess fat if you need to after gaining enough muscle weight to be out of danger of being underweight (continuing to exercise to maintain your muscles is necessary to ensure that weight loss is mostly fat, rather than hard-earned muscle).