There are several different synonyms for the phrase "man up" that can be used in different contexts. One possible alternative is to "be brave," which encompasses the idea of facing challenges and difficult situations with courage. Another option is to "take responsibility," which emphasizes the importance of owning up to one's actions and accepting the consequences of those actions. "Grow up" is another phrase that is sometimes used synonymously with "man up," and it highlights the idea of becoming more mature and responsible. Ultimately, the phrase one chooses to use will depend on the situation and the desired emphasis.