The phrase "rough up" generally means to subject something or someone to physical violence or rough handling. The antonyms for "rough up" would be words that convey the opposite: gentleness, calmness, and peacefulness. Synonyms for these include words like soothe, caress, tame, soften, and relax.
For example, after a tough day at work, one might want to relax and unwind with a soothing massage. Alternatively, a parent may need to calm down a hyperactive child and speak to them in a soft and gentle tone. In general, using antonyms to "rough up" can help bring positivity, relaxation, and a sense of ease to situations that may otherwise be aggressive or confrontational.