Hi! I am sorry but if there is testicular atrophy, nothing can be done. Since you now have swelling in right side, it can be infection, hydrocele or hernia and you should immediately consult an urologist. It is as urgent as that it should have been done yesterday! Take care!
You need to see a doctor. You want to make sure you protect the right testicle for the sake of your health in general as well as your reproductive health. The testicles produce hormones which keep you fit. One testicle will be enough if the left one isn’t functioning. If you are concerned about the cosmetics of the left testicle, if it is no longer functional, you can have it replaced with an implant. If it is functional but just smaller, then leave it alone. We all have differences which make us individuals.
I would suggest that you see a Doctor.