Use an Update query; sample SQL based on your requirement is:
UPDATE tblYourTable
SET YourField = left(YourField, 5) & "\" & Right(YourField,4)
In this example, the field YourField in the table tblYourTable is updated by inserting the backslash between the 5th and sixth characters of a 9 character field, making it 10 characters.
A couple of things to look out for:
(a) Work in a copy of your database, just so you dont burn any bridges if you dont get it right first time.
(b) Add a WHERE clause if you want the update to be selective (I dont think you do in this case).
(c) Replace table / field names as appropriate.
Hope this helps,
Steve Lewy
Solutions Developer
steve@lewycomputing.com.au
(dont cut corners or you'll go round in circles)