Do the following:
cd /usr/local/directadmin/custombuild/
./build update
./build versions
That will tell you what version in the current branch you can update to.
There are also updates to 5.5 and 5.6 that you can perform.
Before you upgrade between versions, make sure you backup your data.
I also make it a practice to 'mysqlcheck -a --repair' each server thoroughly before an upgrade to ensure the process goes smooth.
After upgrading mysql make sure to rebuild PHP:
./build php n