Upgrading more than one release level is supported, but only if you upgrade one release level at a time. For example, upgrade from 5.1 to 5.5, and then to 5.6. Follow the upgrade instructions for each release, in succession.
Direct upgrades that skip a release level (for example, upgrading directly from MySQL 5.1 to 5.6) are not recommended or supported.
In place upgrade of mysql is not an easy task for beginner. If you have option for clean install it will be much easier for you. I have explaind it here, hope it helps.
Now automatically Plesk 12.5 installed MariaDB10 and everything seems to work fine, except in Plesk Add remove component page, it shows that there is no MySQL installed and when I try to install it says it's already installed.