You can cancel an invoice in the customer’s profile. 

To do this, click on the profile of the customer whose invoice you wish to cancel. Here you click on the product for which the invoice was created. 

This will open a transaction overview, in which you will see the "Cancel Invoice" field on the far right.  

Once you click on "Cancel Invoice", a pop-up window opens in which you can confirm the cancellation of the invoice once again. You also have the option to delete the invoice completely, but this is not recommended for tax reasons. 

A cancellation invoice is then created. This can be found in the Accounting menu item. Now you can make changes to the product and create a new invoice, which you can then send to your customer by email.