Hi Sangeeth,

I might be late for this. But it might be helpful for someone else. You can override the validate function as follows:

odoo.define('module_name.POSValidateOverride', function(require) {
    'use strict';

    const PaymentScreen = require('point_of_sale.PaymentScreen');
    const Registries = require('point_of_sale.Registries');

    const POSValidateOverride = PaymentScreen =>
        class extends PaymentScreen {
            /**
             * @override
             */
            async validateOrder(isForceValidate) {
                console.log('POSValidateOverride::validateOrder(): successfully overridden');
                await super.validateOrder(isForceValidate);
                // Add your code
            }
        };

    Registries.Component.extend(PaymentScreen, POSValidateOverride);

    return PaymentScreen;
});

Hope that helps!