looking for an easier way to update

I'm very new to all of this so I'm looking for answers in the SIMPLEST
of terms.  I have a DB that is a multimillion item catalog.  Every month
I get an update to the catalog.  The original catalog is maintained by
someone else running an Oracle system on Windows NT or 2000 servers.
they output a text file and burn it to cd and mail me the updates as a
zip file.  the tables are converted to .txt windows files as tab
delimited text.  Line one is the header and the rest of the file is the
contents of the table.  Currently I have to go in and strip the headers
before I can use the data.  Then I have to delete the current contents
of the table and use COPY from to bring in the new information.  What
I'm looking for is a way to get postgreSQL to update the tables from the
information in the text files and only overwrite what has changed while
leaving thue unchanged stuff intact.  Is my thinking wrong?
Can it do that?
if so, HOW? remember I'm very new  to this so please be as explicit as
possible.

Thanks, David Kucharski