Paying off overdue debt does not instantly make your credit score good. The debt was still not paid per the original agreement with whoever loaned you the money or gave you the credit. So that "ding" doesn't go away just because you pay it off. Only time can improve your credit score. The more of your debt that you pay in accordance with the agreement you signed for that debt, the sooner your score starts rising.
There are things you can do to speed up the process. But you need to pay off the overdue debt first, before even getting into those things.