My iCloud email

Sign out Apple ID from iPhone ( it depends which mail address is signed in ) see the article Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac - Apple Support

And in empty fields enter you already created Apple ID in the past Sign in with your Apple ID - Apple Support

Then , add your mail account in the iPhone Add an email account to your iPhone, iPad, or iPod touch - Apple Support

Note : there is no option for changing mail account in which a web mail address is signed in .