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