How do I request a goodwill adjustment?

I was a longtime DirectTV customer who never missed a payment. When I cancelled my account I was assured by the customer service agent that I had a zero balance. DirectTV however mistakenly charged me for the NFL package which I used to subscribe too annually, though I had cancelled before the regular season had started. At the time I had paperless billing so I never received a bill and I did not log into my account because it was closed and I was told that I had a zero balance. A few months later I received a bill from a collection agency, Sequium Asset Solutions. I immediately called DirectTV, they admitted that I shouldn't have been charged the NFL package and made the necessary adjustments to my balance, though there was a small residual balance which I paid off right away. The collection still remains on my credit report, this is the only such collection on my credit report. Had the DirectTV customer service agent correctly advised me of any residual charges or if DirectTV had sent me a paper bill before contacting a collection agency this charge would have been taken care of right away, I always take care of my outstanding balances promptly. How do I make a request to DirectTV to make a goodwill adjustment to the credit reporting agencies.