Add a back button to the payment step in checkout

To add back button on the payment step, you need to just override two core file:

• magento/module-checkout/view/frontend/web/js/view/payment.js
• magento/module-checkout/view/frontend/web/template/payment.html

You can override them either from theme or by creating custom module.

In payment.html file, add a back button :-

<!--
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<li id="payment" role="presentation" class="checkout-payment-method" data-bind="fadeVisible: isVisible">
    <div id="checkout-step-payment"
         class="step-content"
         data-role="content"
         role="tabpanel"
         aria-hidden="false">
        <!-- ko if: (quoteIsVirtual) -->
            <!-- ko foreach: getRegion('customer-email') -->
                <!-- ko template: getTemplate() --><!-- /ko -->
            <!--/ko-->
        <!--/ko-->
        <form id="co-payment-form" class="form payments" novalidate="novalidate">
            <input data-bind='attr: {value: getFormKey()}' type="hidden" name="form_key"/>
            <fieldset class="fieldset">
                <legend class="legend">
                    <span data-bind="i18n: 'Payment Information'"></span>
                </legend><br />
                <!-- ko foreach: getRegion('place-order-captcha') -->
                    <!-- ko template: getTemplate() --><!-- /ko -->
                <!-- /ko -->
                <br />
                <!-- ko foreach: getRegion('beforeMethods') -->
                    <!-- ko template: getTemplate() --><!-- /ko -->
                <!-- /ko -->
                <div id="checkout-payment-method-load" class="opc-payment" data-bind="visible: isPaymentMethodsAvailable">
                    <!-- ko foreach: getRegion('payment-methods-list') -->
                        <!-- ko template: getTemplate() --><!-- /ko -->
                    <!-- /ko -->
                </div>
                <div class="no-quotes-block" data-bind="visible: isPaymentMethodsAvailable() == false">
                    <!-- ko i18n: 'No Payment method available.'--><!-- /ko -->
                </div>
                <!-- ko foreach: getRegion('afterMethods') -->
                    <!-- ko template: getTemplate() --><!-- /ko -->
                <!-- /ko -->
            </fieldset>
        </form>
        <div class="primary">
            <button data-bind="click: goToPrevStep()" class="back button action primary">
                <span translate="'Return'" />
            </button>
        </div>
    </div>
</li>

In payment.js file, you need to define goToPrevStep() function

/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */

 define([
    'jquery',
    'underscore',
    'uiComponent',
    'ko',
    'Magento_Checkout/js/model/quote',
    'Magento_Checkout/js/model/step-navigator',
    'Magento_Checkout/js/model/payment-service',
    'Magento_Checkout/js/model/payment/method-converter',
    'Magento_Checkout/js/action/get-payment-information',
    'Magento_Checkout/js/model/checkout-data-resolver',
    'mage/translate'
], function (
    $,
    _,
    Component,
    ko,
    quote,
    stepNavigator,
    paymentService,
    methodConverter,
    getPaymentInformation,
    checkoutDataResolver,
    $t
) {
    'use strict';

    /** Set payment methods to collection */
    paymentService.setPaymentMethods(methodConverter(window.checkoutConfig.paymentMethods));

    return Component.extend({
        defaults: {
            template: 'Magento_Checkout/payment',
            activeMethod: ''
        },
        isVisible: ko.observable(quote.isVirtual()),
        quoteIsVirtual: quote.isVirtual(),
        isPaymentMethodsAvailable: ko.computed(function () {
            return paymentService.getAvailablePaymentMethods().length > 0;
        }),

        /** @inheritdoc */
        initialize: function () {
            this._super();
            checkoutDataResolver.resolvePaymentMethod();
            stepNavigator.registerStep(
                'payment',
                null,
                $t('Review & Payments'),
                this.isVisible,
                _.bind(this.navigate, this),
                this.sortOrder
            );

            return this;
        },

        /**
         * Navigate method.
         */
        navigate: function () {
            var self = this;

            if (!self.hasShippingMethod()) {
                this.isVisible(false);
                stepNavigator.setHash('shipping');
            } else {
                getPaymentInformation().done(function () {
                    self.isVisible(true);
                });
            }
        },

        /**
         * @return {Boolean}
         */
        hasShippingMethod: function () {
            return window.checkoutConfig.selectedShippingMethod !== null;
        },

        /**
         * @return {*}
         */
        getFormKey: function () {
            return window.checkoutConfig.formKey;
        },

        goToPrevStep: function () {
            return stepNavigator.navigateTo('shipping');
        }
    });
});

And here is the result (Return button) in bottom.

Thanks..