Beefs up is a phrase which indicates a process of strengthening or fortifying something. There are many different synonyms that can be used to convey the same meaning, including the likes of bolster, enhance, intensify, boost, amplify, augment, and reinforce. All of these words suggest a process of making something stronger or more robust, whether that is a physical object, a mental or emotional state, or an argument or idea. Depending on the context, each of these synonyms can be used to add nuance and variety to your language, making your writing or speech more engaging and effective.