как менять вес в корзине?

<form method="post" class="ms2_form form-inline" role="form">
                                <input type="hidden" name="key" value="{$product.key}">
                                <div class="form-group">
                                    <input type="number" name="weight" value="{$product.weight}"  min="25"
                                           class="input-sm form-control"/>
                                    <span>кг</span>
                                    <button class="btn btn-default" type="submit" name="ms2_action" value="cart/change">
                                        <i class="fa fa-refresh"></i>
                                    </button>
                                </div>
                            </form>

<td style="{$style.th}">{$product.weight} кг</td>