You can't.
You can, however, remove individual addresses from the autocompletion cache
by highlighting the entry when presented in the suggested names list (use
your arrow key to migrate to the entry) and then hitting your Delete key
before you tab out of the field. You can also clear the entire cache. Follow
the instructions here:
http://support.microsoft.com/default.aspx?scid=kb;EN-US;Q287623

Signature
Russ Valentine
[MVP-Outlook]
> I'd like to figure out how to read the system file with
> an NK2 extension so that I can edit out erroneous or
> incomplete entries.
>
> Does anyone know how this file is structured?