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Never tell your boss they are wrong. Never.
What you need to do is make them think they have found out for themselves, and give them a way to change their mind without losing face. A good way to do this is with "new" information. This can be real new information, or a new analysis of existing information, or old information dressed up in a different way, or irrelevant information made to look like it is relevant, or whatever you can think of.
By showing , fact's and figures, and by showing , power point presentation.