Resetting default reply-to and sent from

I followed the advice here to change my default reply to field in Mail https://discussions.apple.com/thread/306861?answerId=1667119022#1667119022

How do I edit Terminal to remove this instruction?

Also, I use an email forwarding address so that if I change ISP or change email account my address stays the same. How can I edit my iCloud account in Mail to display that forwarding email address as sent from when people receive emails from me?