1.) Find them under the INVOICES tab and click the invoice number link to go to the Invoice Details screen. (You can also find the invoice number link on the SEARCH tab).
2.) From the Invoice Details screen, click the trash can in the upper right corner. Then choose "Delete Invoice".
3. If there is a document attached to the invoice, you'll be present with the option to either "Delete Invoice", which deletes the invoice but leaves the document unattached in the Inbox, or "Delete Invoice & Document", which will delete both.