Change the password as follows:

1. Open: /var/lib/roundcube/config/db.inc.php and then look for the password in the command

$rcmail_config['db_dsnw'] = 'mysql://roundcube:password@localhost/roundcube';

2. Copy the password only.

3. Now open: /etc/roundcube/debian-db.php

4. Find:

$dbpass='wrong_password';

5. Replace "wrong_password" with the correct password that you just copied.