Change author name for different posts

Hi all,

I am using the theme celestial-aura. The website I am developing is Here.I(admin) have moved all those posts/stories from an old website to this new website. So the author of all the posts is displayed as admin. But those stories were posted by different other authors in the old website. Is their a way I can change the author name without creating different accounts? I cannot publish the website without giving credit to the original authors. So please help me solve this problem. Thanks in advance.