Treatment for mild narrowing of the acromioclavicular joint with downward sloping acromion?

Depends on if: You're having pain & decreased range of motion. If so, it is probably impingement syndrome of the shoulder caused by weak rotator cuff muscles. A physical therapist can show you exercises which will dramatically help.

Xray Report?: This sounds like an xray report and not a diagnosis for your shoulder pain. Please don't consider any sort of invasive treatment until you have a detailed evaluation and appropriate diagnosis. Learn more here: http://www.Theshouldercenter.Com/shoulderpain/2013/shoulder-surgery/spur-acromioplasty-impingement/.