I think you mean this passage from after the Sorting Hat:

You couldn't change history. But you could get it right to start with. Do something differently the first time around.

This whole business with seeking Slytherin's secrets... seemed an awful lot like the sort of thing where, years later, you would look back and say, 'And that was where it all started going wrong.'

And he would wish desperately for the ability to fall back through time and make a different choice...

Wish granted. Now what?

Harry slowly smiled.

It was a rather counterintuitive thought... but...

But he could, there was no rule saying he couldn't, he could just pretend he'd never heard that little whisper. Let the universe go on in exactly the same way it would have if that one critical moment had never occurred. Twenty years later, that was what he would desperately wish had happened twenty years ago, and twenty years before twenty years later happened to be right now. Altering the distant past was easy, you just had to think of it at the right time.