@canolobato when the actual AMEX transaction for that receipt comes in, you can match it (if the system doesn't) to the receipt and the system will change the payment type for you.
The receipt image that is showing as Cash cannot be changed to an AMEX payment type. The system isn't designed that way. The AMEX payment type is only applied to AMEX transactions that have uploaded into your Available Expenses and is assigned by the system. It cannot be assigned manually.