I have a large table, and I wish to update only a single column. The values for that column is present in a CSV file. I want to avoid a single insert to event column because it would take a long time. I would prefer something like a COPY so that I can directly dump the new values over the older values. But dumping a specific using copy appends it to the end of the table rather than overwriting it. 
Any suggestions?
Suggestion: Insertion is faster than update in DB so can follow below steps:

Load CSV to a temp table.Not all columns are required just primary
key and the column which need to be updated in main table.
Rename main table to main_temp
recreate main table (no records now)
Join main_temp and temp table based on primary key and insert into
main(select particular column from temp table instead from main table)
drop main_temp
Overwrite a column in Postgres from a column in a csv file