Mac Pro el capitan: username and/or pw not accepted: How can I boot up to fix this

I recently acquired a firmware Mac Pro 2009 4.1, loaded with a version update of el capitan.  I immediately changed the admin password but has not changed the admin username, nor have I evolved Autostart and name of user and password until today. NOW, it will not accept any combination of username / password.  How to overcome this gaffe to change the user name and password.

To do this the Apple Downloads are locked on the account who bought them:

Evolution of the property of a Mac of opportunity

Tags: Mac

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