According to Forbes, the best way to inform a new employer about a rescinded job offer is to call the company immediately and let it know of your decision. Be honest about the reason for turning down the job offer.
There are a number of ways to turn down an accepted job offer without ending on a bad note. One thoughtful solution is to follow up with all company employees met during the interview process. Other ways include praising the company's accomplishments and suggesting that you and the employer could work together in the future. You can also recommend a suitable replacement for the vacant position.