Assuming that meta_value is always dd/mm/yyyy and you are looking for yyyymmdd (which isn't clear), try this:
UPDATE wp_postmeta
SET meta_value = CONCAT(SUBSTRING(meta_value FROM 7),
SUBSTRING(meta_value, 4, 2),
SUBSTRING(meta_value, 1, 2));
Run that as a SELECT first to verifying the output before update and make sure it's what you're really looking for.
