How do I modify an order?

[Walrus]

Customer bought wrong cheaper product. He has paid the difference.

 

I now want to change the product to the more expensive one. 

 

In the order page, Prestashop let me delete the cheap product he purchased and downloaded.

 

However, when I try to add a new product, it says, "You cannot add products to delivered orders."

 

I then went into his account and tried to add a product but got same error! 

 

Now, his order page shows he hasn't ordered anything and his total spend is 0.00 amount (not true).

 

How do I undo the mess?

 

TIA.

[Walrus]

Thanks..Didn't work but gave me an idea. I cancelled order, then added a product, and marked it as delivered.