It is very simple. It can be possible only if your page have at least 25 likes. Visit https://www.facebook.com/username/ and select the page you want to assign user name --> Check availability and confirm.
This is Kishore Siluvaikan K. Evergreen learner, I like to blog on the things which I learn from my everyday life. Human by Birth - Blogger by Interest...!
Sharing is caring - Sharing my knowledge here...!