Update twig variable based on user selection.

I am trying to update a variable in the product.twig after a user selects an option, so post page load. I can get the javascript function to fire after the users selects an option and then access the variable then set the variable {% set entry_qty = 'bar' %}. But how do I update the page since twig is server side? Just setting the variable does nothing.

product.twig
<label class="control-label" for="input-quantity">{{ entry_qty }}</label>

OC version not provided.

More information would be needed as the additional code would need to be provided in order to understand what you are trying to achieve.