To preen means to primp and make yourself look better. The opposite of that would be to let yourself go. You could say dress down or go casual. You might also hear the term slob.
The opposite (antonym) of antonym is synonym. An antonym is a word that means the opposite. A synonym is a word that means the same thing, or nearly the same thing, for a given context.
Some words do not have direct opposites [antonyms]. Antonyms do not usually substitute for phrases. The phrase "you are" has no antonym. A contrasting version of "you are" is "you are not".