How to deal with people who think they know design?

How do you deal with people who built something (front-end) and they think they know design when they quite seriously don't?

How do you let them know that changes are required are they are going to happen but without crushing what they've done?

And finally, I guess, how do you generally let people know they've done a crap job but in a professional manner?

Update: I'm now creating the style guides for the company. Lol!