Log into your server with root privileges. Then start a Ruby on Rails console.
Start the console with this command:
gitlab-rails console production
Wait until the console has loaded.
There are multiple ways to find your user. You can search for email or username.
user = User.where(id: 1).first
user = User.find_by(email: 'firstname.lastname@example.org')
Now you can change your password:
user.password = 'secret_pass' user.password_confirmation = 'secret_pass'
It's important that you change both password and password_confirmation to make it work.
Don't forget to save the changes.
Exit the console and try to login with your new password.