My table has col1 and col2 as a joint PRIMARY KEY.

Code:
INSERT INTO mytable ( col1, col2)
VALUES ('val1', 'val2'), ('val3', 'val4'), ('val5', 'val6')
How do I update col2 to the new values if the insert fails. If I use ON DUPLICATE KEY UPDATE I can only set them all to the same value, not individual values.