Click on the drop down arrow on in the top right corner and click account settings.  Under the General tab you can edit your name.  It will take up to 24 hours for the change.  I hyphenated my last name this way.

@donnaduncan Hi Donna,

Facebook allows to choose username that has alphabets, numerics and period(.), nothing apart from these. No hyphen, no underscore no other characters. It also says that you have to choose at least one letter. Please refer to facebook’s guidelines re. choosing username below:
“Use only alphanumeric characters (a-z, 0-9) or periods (“.”), and at least one letter. You may include periods and capital letters to make your username easier to read, but these won’t affect how people find your Page. For example, http://www.facebook.com/fbsitegovernance andhttp://www.facebook.com/FB.Site.Governance go to the same Page.“

@donnaduncan That’s an auto generated username (and actually not username but facebook’s own name assignment to pages) to the pages that either have less than 25 likes or in case if page has more than 25 likes, the admin is yet to set the vanity url/usernmae for that page.For example, when i register a page say for example, social media examiner, then i am assigned facebook.com/pages/social-media-examiner/xxxxxxxxxxxHere these numerical digits are my page’s permanent id number. Once i get 25 likes and once i choose username for my page (say for example smexaminer) then my page’s address is facebook.com/smexaminer.Now as my page’s name is having username (smexaminer) that old numeric id (xxxxxxxxx) and old big url (facebook.com/pages/social-media……….) will still work but will redirect to my new address (facebook.com/smexaminer)So its not username but an auto generated, system originated url at time of page creation. You can’t do anything with that.Hope this helps.