VOID AN INVOICE
You can only void an invoice that is in Draft.
If the invoice is approved or sent then you would need to create a credit note against it and re-create a new invoice.
If the invoice has not been sent to your customer, after you create the credit note, you can select both the invoice and matching credit note and export to Default CSV...

Then select 'Mark as Exported'...
This will flag them both as exported but they will NOT be in Xero (and will not appear on your customer's statement).
Invoice Lock Down
Invoices are locked down for a couple of reasons.
Once an invoice has been Approved and Sent (printed or emailed) it can be exported into your accounting system. Once it is exported then there is no way that you want the ability to edit this inside IQ.
Having the ability to do this can lead to all sorts of issues with the invoice not matching the your accounts package, and could lead to the ability of possible fraud.
The printIQ workflow is that you have the ability to make changes all the way up to printing or emailing the 'approved' invoice. You have the ability of printing the 'Draft' invoice and if you need to send an invoice to your client for approval before sending the final confirmed invoice then we suggest you use this ability.
Once it it marked as 'Sent' or 'Exported', and you need to make changes then your options are to either Credit the entire invoice and reissue a new invoice (These will be different numbers due to auditing issues) and make the amendments that you require.
An other option is that you make the adjustments in your accounting package and control this from within there. Then you will need to managing the payments and credits out of your accounts package as the invoice will not be amending the invoice printIQ.