UPDATE `ps_orders` SET `id_currency` = 2 WHERE `id_currency` = 0;
You can do it like the above but that will set them all to "2" and that's possibly not correct! Have you looked at the ps_cart table? If the currencies look OK in there then you could probably safely run a query to populate the orders with the same currency that the cart used. They may be corrupted too though.
IMPORTANT: Please also remember to take a backup before running SQL on the database that changes data. Then if it all goes wrong you can always go back to where you were before.