User has deposited a payment, but now wants to change the Reference Number on the payment. Core does not allow updating the number and gives an error message:
Deposited payments cannot be updated.
Core does not allow you to update the payments that you have already deposited. So you first need to delete the deposit associated with the payment from the Accounting > Deposits screen by selecting the deposit and clicking on Actions > Delete. Then you can change the Reference Number on the payment as required in the Edit Payment screen. After that, you need to recreate the deposit using the same payments as before. Check the Core Help for details on creating deposits.