How can I edit a payable?

To edit a payable, you will notice a small icon just to the left of the payable amount that looks like a pencil. This is the edit icon. You will be presented with the edit modal box for the payable. Additionally, you can remove a payable with the icon just below the edit icon.

Also, note, if you'd like to edit a payable that's been PAID, meaning it's not longer a payable, it's now a paid expense.  You'll find this under your Accounting (Payables > receipts) page.  Just filter down to the correct receipt, click it to activate it's action drawer and choose the appropriate action.