How do I change website members to login with email?

Hi my client wants to change the login ability for his paid members to login using their email address instead of their user name. I tried various plugins and they didn’t work. The original web developer made some custom edits to the plugin “members” which I’m not sure if it has something to do with this.

Is there anyway by editing some code on the ftp or within word press I can change it so users login with their email instead of user name?

The website is: https://www.papausa.com

Thanks