Beat up is a phrasal verb which means to physically harm someone, or make something look old or damaged. Antonyms of the word "beat up" could be "brand new," "unscathed," "perfect," "pristine," and "intact." These words indicate that something is untouched, undamaged, or new, and are the opposite of the idea conveyed by "beat up." Some other antonyms for "beat up" could be "healthy," "functional," "flawless," "impeccable," "unused," and "unmarked." In essence, anything that represents an unharmed state can be considered an antonym for "beat up.