Here's how to replace a receipt:
Add a new receipt (=correct receipt)
Go to the "Bookkeep > All Payments" or Payments tab.
Select your payment / your colleague's.
Click on "Upload another receipt" or drag-and-drop the receipt on the right sidebar.
Delete a receipt on a payment
Still on that same payment, select the incorrect receipt.
Click on the bin tool at the top left of the pop-up window that appears with your receipt. It looks like this icon on the left:
➡️ It's not possible to delete a receipt unless you add another receipt first.
➡️ It's not possible to delete a receipt if the payment has been exported.