As a rule, you wouldn't. The stored sum would be WRONG if any of the
records which go into the sum were to be changed. Or someone could
open the table, edit the stored sum, and again you'ld have incorrect
data.
If you have a good reason to do so (e.g. you want to capture the sum
as of a certain point in time, or allow it to be edited) you can't use
a Totals query as an Update query. You can use a totals query as an
Append query to add new records to a table; or you can create an
Update query using the DSum() function to do the summing.
