If you have access to both the website files and the database, this is an official - but poorly documented - way to reset the admin/master password on an Umbraco site.
To start, clear the version number inside of the web.config, e.g.
<add key="Umbraco.Core.ConfigurationStatus" value="" />
Next open up the database and, if you’re using Umbraco Version 8, change the password for user
Important: If you’re using Umbraco Version 7 the user you’ll need to update is
Now start the site and it will launch the installer where you can enter a new password (and importantly it won’t “install/upgrade” anything as it’s already up-to-date).
Credit goes to Shannon for posting this little gem on Twitter!