There are several synonyms for the phrase "improve spirits" that can be used to describe a positive change in one's mood. One of the most common synonyms is "boost morale," which implies a swift and significant increase in one's mood. Another great synonym is "brighten one's day," which suggests a small but impactful change that brings a smile to one's face. Additionally, "lift one's spirits" conveys a sense of relief or a weight being lifted off one's shoulders. Finally, "cheer up" is a straightforward and direct synonym that indicates a change from sadness to happiness. Regardless of the specific synonym used, all of these phrases suggest a positive change in one's emotional state.