mysql trigger on update with insert statement
I think that you don't need a trigger at all and - based on your comments - you want an INSERT ... ON DUPLICATE KEY UPDATE. If there is a UNIQUE constraint on (col2), this will work:

INSERT INTO table_name
  (col1, col2) 
VALUES 
  (@col1, @col2)                  -- the new values here
ON DUPLICATE KEY UPDATE
  col1 = col1 + VALUES(col1) ;    -- update the old value by adding the new one