for DATETIME:
UPDATE table_name SET date_col=DATE_FORMAT(date_col,'2013-%m-%d %T');
for DATE:
UPDATE table_name SET date_col=DATE_FORMAT(date_col,'2013-%m-%d');
for CONCAT
UPDATE table_name SET field_name = concat('2011-01-12 ', time(field_name))
for +- INTERVAL
UPDATE table_name SET date_col= date_col + INTERVAL 5 DAY WHERE ...
(you can use MONTH, YEAR, HOUR etc too)