Begin by owning the situation that made the customer angry even if you had nothing to do with it and apologize to him/her. Reassure him/her that you will do your best to offer a solution and if you are unable to, refer him/her to the next authority. Proceed to offer the right solution and make sure the customer is satisfied. Thank the customer for choosing your products/service and make them feel welcome again to choose your company. Wish them a lovely rest of the day.