Click on your username on the top-right corner of the screen and choose “Account & Billing”, or just go to https://apps.risevision.com/billing.
If you already have your invoice, you can do this from the invoice preview page. Click on the “Pay Now” button next to the invoice you want to edit, and then click “Add” or “Edit” next to the PO number to update it.
If you don’t have your invoice yet, you can add the PO to your subscription. Click on the subscription you want to edit, and then click “Add” or “Edit” next to the PO number to update it.
Here’s a quick video walkthrough: