You cannot change the payment mode of the order once it is shipped.
To change the mode of the order before shipping please follow the following steps
Step 1: Go to Orders
Step 2: Go to the order for which mode of payment is to be changed and cancel the order ( to see how to cancel an order click here)
Step 3: Now go to the cancelled order and then click the Clone Order button, change the mode of the payment.
and then click on Submit
You can see a message "Order Added Successfully" on the top right corner, this means you have successfully changed the mode of payment.
Note: Mode of payment can only be changed before the order is picked from the warehouse