If you are the adventuresome type (or perhaps a bit foolish, or brave, depending on your perspective), and can't wait until macOS Sierra is released this fall, here is an article describing how to do the upgrade now. If you should decide to try this, as the article clearly states, back up, back up first. As for me? Initially I thought -- yes, I'll give it a try. Then, I came to my senses and decided to wait until it is released.