Invoices can be deleted directly if they have not been sent to accounting yet.
If they 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, mark the Invoice as Unsent.
This will reset the Sent to Accounting date and you will see the delete button back, allowing you to delete the invoice.
You can then update the relevant line items in the project and re-issue the invoice if required.
NOTE: Deleting Invoice in WorkGuru will NOT remove them form your accounting system. If the invoice has been sent and needs to be updated, you need to manually remove it from the accounting system first.