You can do different exercises with light weights or elastic bands to remove laxity on the back of the arm. I put the exercise sample below, I hope it will be useful, but you should pay attention that these exercises must be performed every day for at least 3 months to get results. Good luck
3 Set 15 Rep per days, Weight with minimum 0.5 kg
Elastic band Exercise: 3 Set 15 Rep per days
Bench Deep: 1-2 Sep To fatigue