Answer
If you need to make adjustments to payment summaries, click the Advanced button on the Payment Summaries screen. You can then change the values as needed.
If you wish to undo any manual adjustments made in this screen, simply click on the Refresh button.
If you have already lodged this payment summary to the ATO, and then need to resubmit your edited version make sure to tick the Amendment? box before resubmitting.