How to insert on duplicate key?

Hi all,

I have a table like this,
CREATE TABLE mytable(
a    varchar(40),
b    text,
c    text,
PRIMARY KEY (a, b)
);

What I want to do is:
     insert a record into a table, and when the record already
exists(according to the primary key), update it.

I know that there is a ON DUPLICATE clause with MySQL, so I'm wondering is
there a quick and clean way to do this in PostgreSQL ?

I have googled and currently the only way I can find is do query first and
then update or insert.

Thanks a lot.

Regards,