Invoices can be deleted directly if they have not been sent to accounting yet.

If the invoice has been sent to accounting, the delete button will not be present.   If there is a scenario that you need to remove that invoice from a project, you are required to remove (delete or void) it from your accounting package first (WorkGuru cannot do this) and then back in WorkGuru, click on Queue Resend to Accounting. 

If it has not been approved in Xero you can delete it in Xero and hit the Queue Resend to Accounting button and edit and then resend.

If it has been approved you will need to void in Xero and issue a new invoice as the existing won't be able to be resent across from WG.