Two pieces of information you left out

#1 - Outlook version
#2 - Whether you want to change this manually or programmatically (going to
assume you mean manually for the purposes of this answer given the <All
Fields> reference in your question

Anyhow - here is a generic answer that covers Outlook 'XP & '2003 at a
minimum (and off the top of my head - this applies to O'2000 as well as
described below - just don't have an O'2000 system handy at the moment to
refresh my memory)

#1) The "FileAs" field is accessible for modification from the main contact
screen (that assumes you are not using a custom form which doesn't include
the "FileAs" field in which case - point #2 applies and likely explains your
overall question/problem
#2) The "FileAs" field cannot be modified via the "All Fields" screen - it's
treated strictly as a "read only derived field" in that part of the Outlook
UI based on the rules set out for it.

General note - if you make a custom change to the "FileAs" (custom change
being defined as entering any value other then one of the <derived options>
appearing in the drop down box) - it will not be over-ridden by any change
to the Full Name (First & Last Name elements). In other words - it stops
acting as a "derived" field. If however, at any point, this gets changed
back to any one of the derived options shown in the drop down box - it then
once again becomes a "derived" field meaning that any change made to the
Full Name will be reflected in the "FileAs" field.

Karl
__________________________________________
Karl Timmermans - The Claxton Group
ContactGenie - Importer 1.3 / DataPorter 2.0
"Power contact importers for MS Outlook '2000/2003"
http://www.contactgenie.com