You can easily change the status of "Invoice not issued" by clicking on it with the left mouse button. You will be able to enter the invoice number in the pop-up window and save the entry. The invoice number is optional, you can leave it blank.
Once saved, the status will change to "Invoice issued" and turn green. If you entered an invoice number in the blank field, it will be visible together with the date and time.
If you want to reset invoice status, click “Detailed view” option. The button “Reset invoice status” will update the task and invoice status will be changed to “Not billed” again.