Delete iTunes Account

My mom has been using account/email(A) for a few years in iTunes. She recently got a new Mac & email(B) & wanted her music on the new machine. But instead of signing into her old account/email(A) she created a new account(B) with the new email address(B).

I need to delete to new account/email(B) & change her old account(A) with email(A) to email(B).