In order to change the password for your Omeka application you need to login to your cPanel -> phpMyAdmin and select your database.

Once you open it you will see all the tables inside the database. Select the omeka_users table and find your admin user that should have value super under the role column of the table.

Open the SQL tab and execute the following query:

UPDATE `omeka_users` SET `password`=sha1(concat(`salt`, 'NEW_PASSWORD')) WHERE `username`='ADMIN_USER';

In the above query replace the ADMIN_USER string with the corresponding one for the admin user and the NEW_PASSWORD string with the password chosen by you.

Your new password will be stored in the database with the SHA1 encryption and you will be able to login to your website with your username and your new password.

More details on how to manage your database through phpMyAdmin can be found in the following SiteGround tutorial:

http://www.siteground.com/tutorials/phpmyadmin/