update question

OK I'm new at this, so please bear with me.
I have a postgres table that gets imported from a text
output of a database - uns_output.
There is another table that was created by hand with
our entire inventory - inventory.  I would like to
update a price column in the inventory table from the
information in the price column of the output table.
I'm quite sure it can be done, but I'm not really a
trained dba and i this is beyond the scope of anything
I've done in the past.  Both tables are indexed on a
common field called pik_num.  Our inventory is only a
small portion of the uns_output file so just importing
ALL of that info would be WAY OVERKILL.
I'm assuming that this would be a multiple step
process, first to find which items in inventory are in
uns_output.  then to update those from the appropriate
line, but I need help with the SQL.  If anyone has the
time or inclination and could help me along or at
least point me to a place where I can find the info,
I'd be forever grateful.  I'm not even sure of any
technical terms where I should begin my search.

Thanks a BUNCH
Dave