WordPress 4.9.5 is a security release for all previous versions since WordPress 3.7
Things to note
It is highly recommended you update your sites immediately in order to apply important security and the additional 25 bug fixes. You can read more by looking at the release notes: https://codex.wordpress.org/Version_4.9.5
If you have not done so already – download the update or simply use your existing installation of WordPress to update your site. Either way – this is an excellent time to make a backup copy of your site(s) before upgrading.