You mean around the armpit, such as behind the armpit and the upper part behind the big arm? Determine the location and then determine the specific means! Leave me a message if you have any questions.
Strength exercise: put mineral water in a bottle, bend your arms forward with the palms of your hands+lift your shoulders horizontally in front, and extend your shoulders and retract your shoulders. If you are in a gym, you can widen your grip, pull down in front of your neck+pull down after strength, and row in a sitting position+row your arms straight. Every movement 15 to 20 times, rest between groups 1 minute.
When you are finished, do shoulder rings and do some aerobic exercise. ...