To change a document to a bill, you will first need to unassociate the existing image from the existing document. The Document will then go back into the Inbox for re-processing.
To unassociate a page from a document:

Click the Inbox tab.
Click History. This displays the Files History table, which lists every document you've sent into Bill.com and processed.
Click the name of the file of whose page(s) you wish to unassociate. The image of the document page will now display, along with the "Process item by: box".
Use the forward and back arrows above the image to navigate to the page you wish to unassociate
When you reach thpage you wisto makchanges to, in the "Process item by: box", click Unassociate
The unassociated pagwill return to your Bill.com Inbox and can be tagged again as a bill, a vendor or a company document.