How does one import records from an excel spreadsheet and overwrite existing
field data for records with updated information and also add totally new
records at the same time?

One table has Policy Number as a primary key with no duplicates and the
other table has Location ID as primary key with no duplicates.  

The importing new records is the easy part and is what I am doing now.  I
now need the ability to update existing fields for records in the database.