Bills can only be edited (including adding invoice attachments) while the bill is still pending approval. Approved bills cannot be edited.
You must be an Accounts Payable User or have custom permission to access Bills to complete the following steps.
Steps:
Go into Accounts Payable → Manage Bills.
Go into your Pending queue.
Select the applicable Bill.
Go to the line item and click the 3-dot options
button.
Choose Edit.
Make changes and Save