Still trying to make it work as originally designed, I see!
Take a look at an Update Query in Access. You can use "Is Not Null" as a
criterion to update (from) a field that holds a non-null value.
My original design suggestion still holds. A "summary table/record" may
seem to help...
... but creating a "summary record" in a separate table only introduces the
potential issue of synchronization. That is, you will have to ensure that:
1) new information to your first table gets properly recorded into the
second; 2) any change to any row of information (old info, but modified)
gets properly recorded into the second, summary table; 3) no changes are
EVER made directly in the summary table, as this would cause it to fall out
of synchronization with what is contained in your "details" table.