The very first thing that you must have for this to happen is the Book of Reincarnation. When you put this item to use, it will enable you to respec your character according to your preference. Not only that, but it will also return all your Experience, or Amrita as it is called in Nioh 2, and also any Skill points that you have used while leveling up. But the Book of Reincarnation does not come without its price. You will be required to meet some conditions if you want to possess the precious item.