An Invoiceable can be edited while they are in a 'to be invoiced' state - via the pencil (edit) icon to the right of the invoiceable.
However, once the invoiceable has been converted to an invoice, and further if any payment has been recorded in the 'Invoices Out' sub-tab - the pencil (edit) icon is not selectable and the Invoiceable can no longer be edited.
The fix for these scenarios are:
For an 'Invoice' that has been partially or fully paid
Go to the 'Invoices Out' sub-tab, click on the 3 dot dropdown to the right of the partially/fully paid invoice, select 'Remove Payments', and confirm the removal via the 'Remove Payments' pop-up.
NOTE: If you intend to redo the payments later, you will want to make a note of the payment amounts and dates.
This will remove all payments, and change the payment status to 'Unpaid'.
Still on the 'Invoices Out' sub-tab, the next step is to cancel the invoice. We do this by, clicking on the invoice's 3 dot dropdown, selecting the 'Cancel' option, confirming this action via the 'Cancel Invoice' pop-up.
Returning back to the 'Progress Payments' sub-tab, we see that the invoice has moved back to an invoiceable, and is again in a 'Pending' state - the pencil (edit) icon is now selectable, and the invoiceable can be edited.
For an 'Invoice' that has not yet had any Payments made
For an 'Invoice' that has not yet had any payments made, we can skip the remove payment(s) steps, and instead:
Go straight to the 'Invoices Out' sub-tab to cancel the invoice. We do this by, clicking on the invoice's 3 dot dropdown, selecting the 'Cancel' option, confirming this action via the 'Cancel Invoice' pop-up.