You can do tricep exercises to build muscle in that lower part of the arm, but you are not going to be able to reduce the fat and skin through exercise.
A classic tricep exercise is a French press. Hold a dumbell with both hands. (If I am able to do bicep curls with a 12-lb weight in each hand, then I will do the French press with a single 12-lb weight.) Lift the weight overhead so that your arms are straight up. Slowly lower the weight BEHIND your head, bending only at the elbows. You want your upper arms stay straight and parallell to your head, elbows pointed straight at the ceiling. The, lift the weight back up over your head.
You can take a look at the website
www.exrx.net for lots of other tricep exercises. Typical ones you might try are dips, kickbacks, and pushdowns.