G,  If there a reason you are trying to store this value.  Why not simply design a query based on
this table.  Add a column to your query and type the following on the Field Row:

NewLastName: Left([LastName],5)

Hope this helps!

No, you cannot modify a table with a normal query.  You would need to
use code to add a column/field to your table.  But if all you want is
the first five characters in the field, what is wrong with using
something like LEFT$(LastName,5) and then querying that?

If the new field should always be the first 5 characters of the surname, you
should not create this field in the table. Instead, do it in a query, by
entering this into a fresh column of the Field row in query design:
   FirstFive: Left([Last Name], 5)

If you need to be able to sometimes assign a value other than the first 5
characters, use a Data Definition Language (DDL) query statement to add a
field to your table, and then an Update query to populate the field.

This code executes the DDL query:
   Dim strSql As String
   Dim db As DAO.Database

   Set db = CurrentDb()
   strSql = "ALTER TABLE MyTable ADD COLUMN MyNewTextField TEXT (5);"
   db.Execute strSql, dbFailOnError
   Set db = Nothing

For help with the Update query:
1. Create a new query into the modified table.
2. Change it to an Update query (Update on Query menu).
3. In the Update row under MyNewTextField, enter:
       Left([Last Name], 5)
4. Run the query, or switch to SQL View (View menu) and copy what you see if
you want to execute it in code.

Unfortunately, the DDL query statement does not set AllowZeroLength to No in
recent versions, as it did in earlier versions. You will need to use DAO
code to modify this property, so you may prefer to use CreateField() instead
of the DDL statement.