How do I massage my swollen arm to reduce its swelling?

Not so easy. If you have lymph edema of the arm you should see a specialist. If you have swelling of the arm for no particular reason you should see a specialist as well because you could have a deep vein thrombosis.
Wear compression sle. Wear a compression sleeve. It is impossible to massage your own are adequately . If you have someone to do it have your physical therapist teach them.