Update field with new string made from characters in existing value
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.