On the device you may void an invoice at Main menu > Database > Sales / Invoices > All orders > Select the order. Keep in mind that the you will always have the order.
Main Menu > Database
Databas > Sales / Invoices
All orders
Select the invoice
Touch the icon with minus anf then Void
If you need to delete the order, you should do the following at main Menu > Settinsg
Sales
Sales > Allow delet orders > On, by default is Off
On the web version you may do the following to void an invoice or delete an order
Dashboard > Sales Orders
Select the invoice
Click on the X to void the invoice, keep in mind that you will always have the base order
if you need to void or delete the order, select it
Click on the X to choose from