Update a users name in SharePoint
When a user has a name change in Active Directory, the User Profile Service does not update the information in SharePoint as seen on the userdisp.aspx page. You can also see this when viewing list/library entries by the user. The Created By field will still display their old name. The following PowerShell command will force an update from AD to SharePoint.
Set-SPUser -Identity 'contoso\UserName' –SyncFromAD –Web https://contoso.portal.com
Reference: Set-SPUser in TechNet.