If you realise by now that tapping on the custom discount again deselects it, it is because there is a deselect as well as an edit mode for the custom discount. The deselect mode can be found here, and the edit mode will be described below.
Regardless of whether the custom discount is on product or bill level, you will be able to edit the discount. Click on the Pencil icon to edit the custom discount.
The custom discount screen will be activated again, with your last entered discount saved. After editing, click Confirm to updated the discount on the product / bill.