Be glad when a person shares their feelings of discontent with you. It implies that the person still has confidence in your interest in the problem and your ability to do something about it.
Think of the situation as an opportunity to shine.
Don’t take it personally or become defensive.
Show genuine care and concern.
Listen.
Let the complaining person vent. Don’t interrupt with defensiveness.
Politely interrupt the upset person and offer guidance.
Apologize assertively when you have made a mistake.