CREATE TABLE aaa (
  a int primary key,
  b text
);

CREATE OR REPLACE FUNCTION update_aaa(newa int, newb text) RETURNS VOID AS $_$
DECLARE
  rc int;
BEGIN
  UPDATE aaa SET b=newb WHERE a=newa;
  GET DIAGNOSTICS rc = ROW_COUNT;
  if rc =  0   then
    INSERT INTO aaa VALUES(newa, newb);
  end if;
  return;
END;
$_$ LANGUAGE plpgsql VOLATILE;

insert into aaa values (default, 'hello');
select update_aaa( 1 , 'hello world');
select update_aaa( 2 , 'good-bye');
select * from aaa;