Login via SSH first. If you don't know how to do this, please read this article: https://support.tilaa.com/entries/20925478
When logged in as root you should perform the following commands:
cd /usr/local/directadmin/custombuild ./build update # Updates source tarballs ./build update_versions # Compiles and installs updates
Please take a glance at http://help.directadmin.com/item.php?id=247 for further details.