Mysql 5.5 is not installed with Plesk 11.5 on Centos.
Does Plesk support Mysql 5.5 ?
How to upgrade Mysql to higher version?
Plesk 11.5 supports Mysql 5.5 , you can see it in Plesk 11.5 release notes .
Plesk autoinstaller relies on the Centos repositories, if Mysql version is available in Centos distributive, Mysql will be upgraded during Plesk upgrade.
The same rule applies for additional yum repositories. If additional yum repository is enabled (for example atomic) that includes higher Mysql version, Mysql will be upgraded during Plesk upgrade components.
In order to upgrade Mysql, login to Plesk as Administrator-> Tools and Settings->Updates and Upgrades->Update Product Components.
The other possible way to upgrade Mysql is launch autoinstaller from command line:
/usr/local/psa/admin/sbin/autoinstaller --debug --select-product-id plesk --select-release-current --upgrade-installed-components