You can change the password for your database under PHP & Database - MariaDB in the One.com Control Panel.
View your current MariaDB database password
If you don’t remember your password, it is possible to find out what the current password is. To show or hide the password, click on the eye icon to the right.
Click on the pencil icon if you want to change the password. Type in your new password and press the rotating arrows icon to save.
Note: Remember to update the login details for any CMS you have installed, like WordPress.