First things first, you can only delete the first or the last sent invoice. Also, deleting an invoice is permanent and the invoice can not be restored after that, so please be sure that you want to delete the invoice before you do it.
If there was a mistake on your invoice, then read this article: What to do if there was a mistake in the invoice I sent.
You can delete invoice drafts freely.
First, go to your invoice list from the dashboard by clicking Show invoices:
Then you can view all the invoices you have ever created in Isolta and their statuses (Sent, Draft, Overdue, Paid):
Click and open the invoice you wish to delete. After opening the invoice, click Tools at the upper right corner of the invoice) - Delete invoice (draft):
Click Delete invoice (draft). After that, the program will ask you one last time whether you're sure you want to delete the invoice. If you are sure then choose OK:
Deleting sent invoices happens the same way as deleting invoice drafts.
If you can't choose the delete invoice option, then it's probably somewhere in the middle of your invoices. In that case read this article for further instructions.
PS! Make sure you open the invoice from your invoice list. You can't delete an invoice from the invoice list.