They must repent, seek forgiveness from Allah, and be regretful.
They must also make up this day.
They must offer an expiation.
Expiation would be:
Freeing a believing slave, and if not feasible, then one should fast two consecutive months, and if not able to do so, then one should feed 60 poor Muslim people.
Narrated Abu Huraira (ra): While we were sitting with the Prophet (pbuh) a man came and said, “Oh, Allah’s Messenger! I have been ruined.” Allah’s Messenger (pbuh) asked what was the matter with him. He replied, “I had sexual intercourse with my wife while I was fasting.” Allah’s Messenger (pbuh) asked him, “Can you afford to free a slave?” He replied in the negative. Allah’s Messenger (pbuh) asked him, “Can you fast for two consecutive months?” He replied in the negative. The Prophet (pbuh) asked him, “Can you afford to feed sixty poor persons?” He replied in the negative. The Prophet (pbuh) kept silent and while he was in that state, a big basket full of dates was brought to the Prophet (pbuh). He asked, “Where is the questioner?” He replied, “I (am here).” The Prophet (pbuh) said (to him), “Take this (basket of dates) and give it in charity.” The man said, “Should I give it to a person poorer than I.” The Prophet (pbuh) smiled till his premolar teeth became visible and then said, “Feed your family with it.” [Bukhari and Muslim]