How to Merge a client who has changed her name

How do I merge the historical records of "two clients" that are actually the same client?  I have a client who has changed her last name due to marriage and I want to maintain her history, adding it to the new name that she has created here.  Yes, it would have been easier for her to edit and change her last name, but instead she created a new account.  I want to move all of her records from the old account into the new account and then delete the old account.  HOW?