Deleting test invoices

Hi guys hope you can help
just got hold of billable, just what i wanted.
any way while trying it out i made up some invoices to play around with.
now i have registerd and using it properly
HOW do i remove the test invoices that i have created.
I dont want to re install app as i also have printed out some real costomer invoices.

hope you can help.

regards
AGURU