update table from csv file

I have been using COPY FROM to do a mass import of records from CSV files into a new database. I have discover however,
asmall number of records ( a few thousand) in one of the files that contain new data that needs to be added to the
database,but on rows that have a primary key and have already been inserted (so I can't use COPY FROM because it
violatesthe primary key).  

If the structure of the table is

id    data1    data2    data3

and the structure of the CSV file is

id    data1    data2    data3

and I need to update all the rows in data3 where the id = id.

I have created a temporary table and used COPY FROM to load the update data into the temporary table. I seem to be
stuckhowever. I thought I should be able to use the UPDATE command to update all columns and all rows the table.id =
tmp_table.id

Something like:

UPDATE table FROM tmp_table WHERE table.id = tmp_table.id;

Or am I completely off course?