If your stock is held through a brokerage firm, the changes can be made with the person servicing your account.
If you are a "shareholder of record", meaning you hold the stock directly in your name, you need to contact SM Energy’s stock transfer agent:
Computershare Investor Services
350 Indiana Street,
Ste. 800
Golden, CO 80401
Phone: 303-262-0600
Fax: 303-260-0700
Or, visit their website for useful information and change forms: www.computershare.com