The phrase "act upon" refers to the action of taking a step or making a decision based on a specific situation or set of circumstances. There are several synonyms that can be used to replace this phrase, including "react to," "respond to," "implement," "execute," "follow through," "conduct," and "undertake." Each of these synonyms has a distinct nuance and implication, though they all convey a similar idea of taking action in response to something that requires attention or consideration. Selecting the appropriate synonym for "act upon" depends on the context and the intended meaning of the sentence or phrase.