If you want to release it as a true second edition, contact KDP beforehand and give them the ASIN of the original book. Let them know that it's being published as a second edition and you want your reviews, ranking, and everything changed over to it. They'll reply with further instructions. Then publish the second edition as if it were a new book. Once it's live, reply to their email and give them the ASIN of the new second edition. They'll move everything over in a few days. Some readers might want to read the new version and they will buy the second edition, even though they've already bought the first.
If it's not drastically changed, like moving chapters around, you can just upload the new manuscript file to the existing book and you're done.