Magento 2.1.3 – Replace $0.00 with Free for Free Shipping

File path should be below

vendor/magento/module-tax/view/frontend/web/template/checkout/summary/shipping.‌​html file

Add your custom <span class="title">Custom Text</span> text like below example

<!-- ko if: isExcludingDisplayed() -->
    <tr class="totals shipping excl">
        <th class="mark" scope="row">
            <span class="title">Custom Text</span>
            <span class="label" data-bind="i18n: title"></span>
            <span class="value" data-bind="text: getShippingMethodTitle()"></span>
        </th>
        <td class="amount">
            <!-- ko if: isCalculated() -->
            <span class="price"
                  data-bind="text: getValue(), attr: {'data-th': title}"></span>
            <!-- /ko -->
            <!-- ko ifnot: isCalculated() -->
            <span class="not-calculated"
                  data-bind="text: getValue(), attr: {'data-th': title}"></span>
            <!-- /ko -->
        </td>
    </tr>
<!-- /ko -->

Run below commands

php bin/magento setup:upgrade 
php bin/magento setup:static-content:deploy