Mattermost Database Password changed

Hey,

we want to use Mattermost with our GitLab installation.
But it appeared it is not working although the config url is properly set and DNS on the lokal network configured.
In the mattermost event log, I can see errors that mattermost can not connect to its database due to credential missmatch.
How can I setup the mattermost database credentials or reset it to its default value?