Problem scenario: When you bring up the web UI for GitLab CE (Community Edition) for the first time, you are prompted to enter a new password twice. This password will be for the admin@example.com username. If someone else set it up and failed to provide you with the username, and the back end email has not been configured, follow these directions.
Prerequisite: You must have root access.
Solution: As root, enter an interactive console (another set of prompts) and change the default user's password:
1) gitlab-rails console production
2) user = User.where(id: 1).first
3) user.password = 'ciNewPassword'
4) user.password_confirmation = 'ciNewPassword'
5) user.save!
#Remember to change ciNewPassword in steps 3 and 4 to the new password you want.
Mostly taken from:
http://doc.gitlab.com/ce/security/reset_root_password.html