Okay I figured it out.
Here is the code for future generations who are going to need similar help.
$Cart = $this->context->cart; // can be achieved through cookies aswell, a matter of opinion
$Cart->updateQty($quantity, $id_product, $id_product_attribute = null, $id_customization = false, $operator = 'up', $id_address_delivery = 0, $shop = null, $auto_add_cart_rule = true);