This plugin handles Joomla's default user authentication, but allows an admin to login in as any user with just the admin password.
Warning! The admin password should be very strong (at least 20 characters recommended). Do not forget to disable if not needed.
Configuration is easy, just enable the plugin with Menu
Extensions -> Plugin Manager and enter an admin user name in the plugin settings section (it could be any user, but for obvious reason you should only enter your admin user). Now that admin user should be able to login in as any users by entering the user name (not the admin user name) and the admin password.
This plugin is still "beta". Please don't forget to backup and test in a sandbox system first :-)