Twenty Sixteen, the default theme, is available for download on GitHub and the WordPress theme directory. As Tammie Lister stated, Twenty Sixteen will be developed as if it were a feature plugin and will merge into WordPress core later this year. The users can easily update to new versions as soon as they become available by installing and activating Twenty Sixteen. Up to this point, Twenty Sixteen has 23 issues and 27 pull requests on GitHub.
If you are not familiar with how GitHub works, make sure you follow this Contributing to Twenty Sixteen and check out at Twenty Sixteen Now Available for more details.