What is the best way to handle an angry customer in order not to lose him or her?

first of all you let him vent  me listen to his frustration, during my quite time listening I gather information regarding the issue..

second, I summarize what  he/she said  in my Own words,  and a sure help (to show Empathy)

find alternatives to to solve the issue....

first  put  mind the customer is always right,listen to the complaint and attend to the customer in a calm situation and if necessary apologise.

Be calm the customer down by listening to his complain and resolve his issue as soon as possible.