I get the following error: This transaction has been deposited. If you want to change or delete it, you must edit the deposit it appears on and remove it first.
Transactions in deposits are considered "closed" (usually deposits made from Undeposited Funds but not always) The error would indicate that you are attempting to change a total of a payment already on a deposit. To get past this you must open the deposit, and in some manner remove the payment for editing, change it and then add it back. If the source was Undeposited Funds you can, in the deposit window, delete the payment or the entire deposit and all payments will revert back to UF. Then you can edit and re-deposit.
When you remove the payment you must "save" the deposit before you can edit. Consequently, if the deposit has been reconciled the trick is to add a dummy payment in the deposit screen before removing the payment to be edited. Then once done editing, add the correct payment back in, and delete the dummy placeholder payment.