Magento 2.3 - Modify Qty and also the variation using observer

ON add to cart option on product detail page . i am listening the event called checkout_cart_product_add_before and i created a custom observer for this.

It is getting called properly but i want to change the qty and variation which is getting passed from the request paramets.

$observer->getEvent()->getInfo();

if i print this i am getting the data properly.But i need to update the values which are in this object, based on some conditions.