Hi,
You need to edit the file name "settings.inc.php" and change the password of the database on PrestaShop version 1.5 from _DB_PASSWD_
Below is the code example.
define('_DB_SERVER_', '127.0.0.1');
define('_DB_NAME_', 'test_ZGFNn');
define('_DB_USER_', 'test_RvEVe');
define('_DB_PASSWD_', 'AGdG5ea6W');
Also, if you want to change the database password, then you can do using the Cpanel of the web hosting account inside the MySQL section.