As the field is stored as
varchar(255) in that table (that is, it's stored as a string, rather than a special date format), you'll need to slice it and then join the chunks together.
Assuming that the original date is dd/mm/YYYY and the required format is YYYYmmdd then you could do something like this:
UPDATE wp_postmeta SET meta_value = concat(substring(meta_value,7,4),substring(meta_value,4,2),substring(meta_value,1,2))
This is using the
substring function to split the original string, the format of this command is:
substring(string to split,starting position in string, length of required substring)
This isn't the only format, more examples are here - http://dev.mysql.com/doc/refman/5.0/en/string-functions.html#function_substr
Then we use the
concat function to join the 3 new strings as one (details on that one here - http://dev.mysql.com/doc/refman/5.0/en/string-functions.html#function_concat)
But you'll need to be very careful running that update as it is. At the moment it will do that for every row in the table. So if you have a row with post_meta='Stuart Moore' it will be updated to 'MooarSt' which probably isn't what you want. So make sure you use a where clause to restrict which rows you're modifying.