You can delete an invoice in the following manner:
Invoices List:
Navigate to the Invoices List
Select and check the invoice you’d like to delete
Click on the 'delete' icon
Customer Screen:
Navigate to a particular customer
Click on the Invoices tab
Select and check the invoice you’d like to delete
Click on the 'delete' icon
Note: Deleting invoices is a permanent action and cannot be undo.