Try this code on a button click:

for line in self.order_line:
if line.id in self.order_line.ids:
line_ids = self.order_line.filtered(lambda m: m.product_id.id == line.product_id.id)
quantity = 0
for qty in line_ids:
quantity += qty.product_uom_qty
line_ids[0].write({'product_uom_qty': quantity, 'order_id': line_ids[0].order_id.id})