Hi,
From what I can see, the problem is that there is a second input with the same name on the page:
take.ms/0FsId
And when you change the quantity, the checkout form is submitted and both inputs value are sent, but the second one which doesn't change, overrides the first one and thus you don't get to see any change.
Do you have the same issue with the default template of Joomla ?
Do you have a folder templates/YOUR_TEMPLATE/html/com_hikashop/checkout on your website ? If you rename that folder, does it fix the problem ?