Looking at the menu you may conclude that once a sale is made there is no way to modify it. As of the date of this post the ability to cancel a sale is in the future. However, there is a way to exchange an item for the customer.
Let’s say a customer buys item 1 then in a couple days decides they like the finish of item 2 better. Being focused on customer service you agree to make the swap. If you just physically swap the items your inventory will be wrong. This can be quickly remedied in Its A Party by entering a new order for the customer then:
Enter item 1 but enter the quantity as negative, i.e., -1 (assuming they bought one of the item)
Enter item 2 with a positive quantity.
If there is any difference in cost, that will be shown.
That’s it. Your customer is satisfied. Your inventory is correct and you have an audit trail.