The phrase "lighten up" means to relax and not take things too seriously. There are several synonyms that can be used in place of this phrase to convey the same message. One option is "chill out," which suggests taking a step back and calming down. Another synonym is "loosen up," which implies releasing tension and letting go of stress. "Take it easy" is also a common phrase that can mean to relax and not worry too much. Finally, "unwind" can be used in a similar context, suggesting the need to let go of tension and de-stress. Using any of these synonyms can help convey the idea of taking a break and not being too serious or uptight.