How can you treat a damaged meniscus?

Depends. It will depend on the degree of damage. If it is not fully torn, you may be helped by physical therapy, brace wearing, gait retraining etc. If it is fully torn and there is a lot of arthrtis, your best bet would probably be surgery.
Meniscus. Usually scope partial medial meniscectomy or repair if clinically significant locking, painful clicking, or buckling.