Can I replace a receipt that has been uploaded?
This article explains that yes, an uploaded receipt can be replaced in ExpensePlus.
If the receipt is yet to be checked, the User who submitted it, or a Finance User, can simply go to the purchase, select 'I'd like to replace this receipt' and attach a new file. This file will override the original file that was uploaded/emailed.
If you accepted or approved a receipt by mistake and it has already been checked, only a user with the role of Finance Team will be able to replace the uploaded receipt.
A user with the role of Finance Team can simply go to the purchase, select 'I'd like to replace this receipt' and attach a new file.
Receipts that are replaced will go back through the receipt check process (but they won't require approval again if the purchase was previously approved by the budget holder, unless the budget details have been changed too).

For security, when replacing a receipt, new receipts can't be emailed in directly. They can only be replaced by being uploaded or photographed. This is to prevent receipts from being overridden.
To help you better understand the Receipts module as a whole, please visit the module overview page here.