Good to see you here, Tammy132.
I understand you need to create a change order for a customer in order to receive a purchase order for the additional work added. However, the change order feature isn't available in QuickBooks Online.
As an alternative, we can open the purchase order you need to update and make the necessary changes to reflect the additional work. After that, utilize Your Message to Vendor or the Memo field to provide details about the change order.

This will allow you to document the change order and receive the revised purchase order from the customer.
Please let me know if you have any other questions or if there's anything else I can assist you with regarding this workaround in QuickBooks Online, Tammy123.