both email and username i think easy to do, maybe im wrong but i think just two places to change
1.includes/classes/rlAccount -> function login
find code
PHP Code:
Only registered members can view the code.
change sql line to
PHP Code:
Only registered members can view the code.
2. 1.includes/classes/rlAccount -> function isLogin
find code
PHP Code:
Only registered members can view the code.
change to
PHP Code:
Only registered members can view the code.
Also to change text maybe to say that you may login also with email.
Login only with email also can be done, but changes are different.
Easiest way without remaking lot of things is to hide username field on registration step, copy email to username field in the database and some other changes and corrections.