How do you get rid of fat under arm?

The same way you get rid of any fat- eat right and exercise. You can't specifically target your armpits. You'll lose fat from your entire body, some of which will be from your armpit areas. If you feel you have a disproportionate amount of fat in your armpits, you could consider Liposuction- though it's expensive and chances are you'll eventually gain it all back.