React 16 includes a number of small breaking changes. They only affect uncommon use cases and we don’t expect them to break most apps. So if you have apps running in 15.6 without any warnings, upgrade to 16 should be easy for you.
Reference: https://reactjs.org/blog/2017/09/26/react-v16.0.html
Changelog: https://github.com/facebook/react/blob/master/CHANGELOG.md
These are the things you need to be aware of when updating from React 15 to 16: