1. Go to Invoices.
2. On the left-hand side, select the invoice you want to delete.
3. On the right-hand side, click More.
4. Select Delete.

Note that if the invoice is marked as Paid you first have to mark it back to Sent (not paid) before you are able to delete the invoice.

Once an invoice has been deleted, it can not be restored. You can however make a copy out of it:
1. Go to Invoices.
2. Choose Show deleted invoices on the drop down menu on top of the screen.
3. Choose invoice on the left.
4. Click on More on the right.
5. Choose Copy as new.

Due to accounting reasons, it is also not possible to delete the invoice completely and for example use the invoice number again. The next invoice will have the next number in the series.