Porasi pachkhan (pachhan) is a vrata/practice related to food rules in Jainism. If you have taken food after midnight (12 pm at night), it would mean you have not followed the rule you had set for that vrata for that day. In such a case:

• Reflect on the lapse with a sense of remorse (internal pratikraman) and renew your intention to follow the rule strictly from the next meal.
• There is no need for a special ritual to “undo” the lapse; simply resolve to continue with your vrata correctly on the next day and maintain mindfulness in eating choices.
• If you regularly observe porasi pachkhan as part of a vrata, use the next opportunity to observe it properly, and consider adding a brief pratikraman at the end of the day to reaffirm your commitment.

If you’d like, tell me the exact form of porasi pachkhan you’re following (Digambar or Shwetambar variant, and whether it’s a daily or seasonal vrata), and I can give a more precise, tradition-specific guidance.