To reset the password for the admin user, use the following commands:

1. Navigate to the root directory of the Redmine application, for example:

cd /opt/redmine-5.1.3

2. Switch to the user under which the application is installed

2.1 For root:

su -

2.2 For another user, for example, redmine:

su - redmine

3. Find the user and reset the password

RAILS_ENV=production bundle exec rails c

user = User.where(email: 'users-mail@domain.tld').first

user.password = 'new-password'
user.password_confirmation = 'new-password'

user.save!

That's it! Now you can log in via the web using the new password.