Hi @mawomommoh,
First, please don't use the standard credentials as it's a big security problem.
Deleting $SPLUNK_HOME/etc/passwd is a good way to restore the default admin credentials.
After that, I would totally recommend to change the default password to anything else by typing this command on the CLI:
splunk edit user <username> -auth admin:<admin_password> --newpassword <password>
If you're using a 7.x Universal Forwarder, I guess you had to enter an admin password while/after the installation, so admin/changeme won't work.
See also here:
https://docs.splunk.com/Documentation/Splunk/7.1.1/Security/Changeapassword#Change_a_user_password_in_the_command_line
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