A new Redmine site starts with a built-in administrator account so the first operator can finish setup in the browser. The bootstrap account uses the public default password, so changing it before adding projects, users, or email settings prevents the default credential from remaining an administrator login.
The current Redmine release redirects the default admin account to My account → Change password after the first successful sign-in. The form asks for the current password, the new password, and the confirmation, and Redmine enforces the configured minimum password length before accepting the change.
Built-in Redmine passwords can be changed in the web interface, while LDAP or other external authentication sources manage passwords outside Redmine. A completed change rejects admin as the old password and accepts only the new administrator password.
https://redmine.example.net/login
Replace the hostname with the URL for your Redmine site.
The default account has administrator access. Use the default password only long enough to replace it.
Redmine requires at least 8 characters unless the site administrator changed the minimum password length.

Invalid user or password confirms the default password no longer works for the administrator account.
The top bar should show Logged in as admin.