The Debian 9.9 released with lots of bug fixes and many critical fixes. There is no new functionality added but multiple security issues are resolved.

The Debian project is pleased to announce the ninth update of its stable distribution Debian 9 (codename stretch). This point release mainly adds corrections for security issues, along with a few adjustments for serious problems.

Please note that the point release does not constitute a new version of Debian 9 but only updates some of the packages included. There is no need to throw away old stretch media. After installation, packages can be upgraded to the current versions using an up-to-date Debian mirror.

If you have already installed Debian 9. Just update and upgrade your system with following command

sudo apt-get update

sudp apt-get upgrade

To see list of bug fixes and changes you can refer

https://www.debian.org/News/2019/20190427