That's not criteria -- you are not searching for it. It's an update query, and it goes in the Update To field.
UPDATE TableWithFaxNumberYouWantToUpdate SET FaxFieldName = 6 & [FaxFieldName];
Make a new query, and drag in the table with the fax field you want to update. Change the query type to Update, and in the Update To, add 6 & [FaxFieldName], where FaxFieldName is the name of the field storing your fax number (the same field you dragged down).