While the simplest and most direct method for editing xrefs is to open the source file for the referenced drawing, there is a convenient alternative.
If you need to edit the model space objects in an xref, you can access the xref or a nested xref directly from the External References palette or with the XOPEN command. Select the xref and, using the shortcut menu in the palette, open its source file in a separate window. After you save the edits, close the source drawing, save back the edited xrefs in your original drawing, and resume working.