you'd probably be better off posting in the AutoCAD .NET forum:
http://discussion.autodesk.com/forums/forum.jspa?forumID=152
However, I'll take a stab:
Get the path from the PathName property from the BlockTableRecord for the xref.
Create a Database:
Database db = new Database();
db.ReadDwgFile(); // need to use proper overload to pass in the path
edit the db like any other, then save its changes and dispose of it.
There is:
Autodesk.AutoCAD.DatabaseServices.Database.ReloadXrefs(Autodesk.AutoCAD.DatabaseServices.ObjectIdCollection);
which you'd probably pass in an ObjectIdCollection of all the blocktablerecord objectids that you need to reload, in this case, it would only be the one that you are modifying. You might need to then tell the view to regen.
Martin Schmid
Product Line Manager
Mechanical Detailing and Electrical Design
Architecture, Engineering, and Construction
Autodesk, Inc.