What exercise to do to firm that under arm flab . rest of arm is fine?

Velashape, arm lift: Unless you are really young, exercises will not firm up this skin. Back of atm is triceps muscle so theoretically if you exercised enough of this (tricep dips, and others) you may build enough muscle to fill in this space, but you'd be talking pretty buff arms! if you can get some Retin-A to massage, and a needling device like dermaroller it will help firm up the skin too. Otherwise surgery.

"Flabby" Arms: Without seeing photos of your arms, it's difficult to say for sure, but often times no matter how much one exercises, excess skin of the upper arms remains. Plastic surgeons can perform an operation called a brachioplasty to remove this excess skin. The trade-off is a permanent scar, but the benefit is an improved contour of the arms.