At least in some versions, right-clicking a TOC and choosing to edit the field is not reliable. You can edit a TOC field directly in the text if you first choose to display field codes (use Alt+F9 as a toggle).
However, in this case it will be easier to set up appendix numbering from the start as discussed in the article to which Charles has already linked:
http://shaunakelly.com/word/numberin...ppendixes.html.