There is no "prebuilt" way of changing the login name... You should also note that, I believe, any posts are assigned by login name so if you where to update the login name the users posts woudln't be associated with her account.
You should instead update their publicly displayed name to be the proper information, this is good from both a security and maintenance standpoint as it'd be more difficult for someone trying to break into this users account as they'd be trying to login with a user account that wasn't valid and it'd be the easiest route to work with...
You can easily allow the user to update their display name from the control panel (or use a modification from UBBDev which would allow you to update the PDN from the controlpanel).
