how to merge a table from another DB

I have a bit of a dilemma, I'm fairly new to PG, and not quite sure how
to do this:
I have a DB that has been basically 'forked' - a copy of the original
was made for a handful of users to play with without messing with the
live DB. Now, they would like to know if the changes they have made to
the forked db can be merged back in to the live one. A change to one
column of every row in a certain table. How can I update just that field
in every row of the db without harming the existing data already in that
table?

thanks in advance.

-Todd