How To Delete Facebook Username On Macbook How To Delete History...?

How to delete history...? - how to delete facebook username on macbook

Okay, so I have a MacBook, and when I signed on Facebook to full name for me when I write a letter. For example if my name / e-mail has been completed fjfjf@yahoo.com, the second type-if the toolbar on the full e-mail. I do not like and want to somehow delete the e-mail / username, you can write in full, please do not ask why it's a long story haha. How do I remove it? I clean my story, and I tried the button "Delete", but it will not work! Any tips?