Changing Author?

I was told that I could use this plugin and as admin I could post the article but then I could change which author or contributor i wanted to show in Fancier Author for that specific article?

how do I do that?

I created the other user on my site and have his bio ready.. I just cant figure out how to set him as the author of the article

https://www.ads-software.com/extend/plugins/fancier-author-box/