To change a Supplier Rep's name or email on an order, click the associated field on your order, make the necessary edits, and click Save. This will save for all future PO's.
If you do not need to place an order with them at this time but wish to edit the contact information, you can create a draft order with an item from that supplier. Change the email on that order, save the changes, then delete that draft order (be sure not to send it). It will save the updated email and apply for all orders for that supplier moving forward.