такой вариант
UPDATE `table_name` t1, `table_name` t2
SET t1.`field_name`= REPLACE(t2.`field_name`, '&', '-')
WHERE t1.id = t2.id
такой вариант
UPDATE `table_name` t1, `table_name` t2
SET t1.`field_name`= REPLACE(t2.`field_name`, '&', '-')
WHERE t1.id = t2.id