How can a person atone for mistreating his parents after their death?

Disrespecting one's parents (Uquq al-Walidayn) is one of the gravest sins, encompassing any word or action from a child that harms their parents without a valid religious justification. It is possible to repent from any sin, no matter how great. Allah Almighty says: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'"

Repentance has three conditions: ceasing the disobedience, regretting the action, and resolving never to return to it. If the parents have passed away, then regret alone is considered repentance. The child can then strive to show them kindness and dutifulness after their death by: frequently supplicating for them, performing good deeds whose reward reaches them, such as charity and Hajj on their behalf, and honoring their friends.