How do I change the email address on a private account?
How to change supplier contact for the Purchase Order(PO) which was sent to private account?

Only your customer can change the email address on a private account. Please contact the customer directly to request an email address update, or ask them to submit the purchase order or trading relationship to your existing account without using a private one.

For more information on how to change the email address on a private account on buyer side, please refer to How do I change the email address of a supplier's private temporary account?

SAP Business Network for Procurement & Supply Chain > Administration (Buyer & Supplier Account Administration) > Link accounts
SAP Business Network for Procurement & Supply Chain > Transaction Documents > Transaction (Purchase Orders, Ship Notices & Goods Receipts)