See at PostSecret:
How do you repent when you don’t believe in God?
I would say there are simple answers to that, even if the word “repentance” usually has a more religious connotation.
If you did something wrong, then make amends for it. Promise yourself you’ll never do it again. Apologize to those affected by your actions. Do something good to make up for whatever you did that was bad. Think more carefully in the future so you don’t make the same sort of mistake again.
How would you repent?