The XmlDataSource control doesn't have the built-in API to edit an XML File specified via the XmlDataSource's DataFile property.
So, it's necessary to edit the XML File via the built-in .NET Framework's classes: XmlNode, XmlAttribute, etc.
Note: As the XmlDataSource control doesn't have the built-in API to edit the XML File, the ASPxGridView control renders its editor as ReadOnly. So, it's necessary to enable the ASPxGridView's editors explicitely by handling the ASPxGridView's CellEditorInitialize event.