Believing they are better / Thinking less of others
Before You Begin
Pause and breathe. Even if this shouldn't be happening... accept that it is and embrace it with composure and calmness.
Calmness is the only way forward because without calm, there is no possibility for receptiveness and connection. Your child won't give to you any more than they see from you.
Never forget that a strong connection is fundamental, as there will be no correction in them if there is no connection with you.
The Core Problem
Trying to feel better about themselves, because they are insecure.
What Will Make It Worse
Saying something intentionally harsh to "bring them down a notch" will accomplish little and lead to new behavioral problems down the road.
What The Child Needs
1. A better perspective: Your child has likely benefited from some (or a lot of) good fortune in their lives. Kindly remind the child that what they are feeling superior about isn't entirely their doing, and they need to acknowledge and have gratitude for what was given to them.
Additionally, point out that people are not 100% better or worse than another person. Help the child recognize that everyone has strengths and weaknesses, and "better" than another person in one area doesn't make you an overall better person.
Lastly, teach them that there are always going to be people better and worse off than you. Not everyone hits life's high and low swings at the same time, so while right now you are at a high point and another person could be going through a low point, it may change in a year and they would then be in the position to look down on you.
2. Healthy self-esteem: Encourage them to develop a healthy sense of self-esteem that is not based on feeling superior to others, but on valuing oneself for one's unique qualities, strengths and accomplishments.
How To Have The Conversation With Your Child
Find a calm and private moment to initiate the conversation, where both you and the child can focus without distractions.
Begin the conversation with empathy, expressing that everyone wants to feel like "the big dog" or "at the top" but that feeling doesn't come from not treating others with respect and having humility.
Tell them that you want them to feel good about themselves and bring attention to what they do well, but they haven't been going about it the right way and you'd like to guide them to a healthier way of arriving at that feeling.
Next talk through the pointers under "What The Child Needs".
Close by sharing that you are proud of them for being them, not because of how they compare to others.