WordPress 5.1.1 is the security and maintenance release available to update starting March 12, 2019.
The release brings several fixes and enhancements including a couple of security fixes regarding how comments are handled.
You can update to the latest release 5.1.1 by downloading the software or directly from your WordPress dashboard.
Navigate to Dashboard → Updates and click Update Now.
If your website supports automatic background updates, it should have updated automatically to 5.1.1 version and you should have received an email notifying you of the update.
You can learn more about the release in the official WordPress.org development blog.