First create a module using free module creator tool called silk software

Magento 2 Module Creator Tool Using Silk Software

and enable using following commands below.

1. php bin/magento module:enable Tw_Cart

2. php bin/magento setup:upgrade

3. php bin/magento setup:di:compile

4. php bin/magento setup:static-content:deploy

create events.xml in the path:- app/code/Tw/Cart/etc/frontend/events.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="checkout_cart_product_add_after">
        <observer name="Tw_Cart_Addtocart_Customprice" instance="TW\Cart\Observer\CustomPrice" />
    </event>
</config>

create CustomPrice.php in the path: app/code/Tw/Cart/Observer/ & keep the below code

<?php
    namespace TW\Cart\Observer;
    use Magento\Framework\Event\ObserverInterface;
    use Magento\Framework\App\RequestInterface;
    use Psr\Log\LoggerInterface;
    class CustomPrice implements ObserverInterface
    {
    
    public function execute(\Magento\Framework\Event\Observer $observer) 
    {
            $writer = new \Zend\Log\Writer\Stream(BP.'/var/log/stackexchange.log');
            $logger = new \Zend\Log\Logger();
            $logger->addWriter($writer);
            /* Code here */
            $quote_item = $observer->getEvent()->getQuoteItem();
            $price = 400; //set your price here
            $quote_item->setCustomPrice($price);
            $quote_item->setOriginalCustomPrice($price);
            $quote_item->getProduct()->setIsSuperMode(true);
            $logger->info("success !!!!");
    }

}

First create a module using free module creator tool called silk software

Magento 2 Module Creator Tool Using Silk Software

and enable using following commands below.

 1. php bin/magento module:enable Tw_Cart
 2. php bin/magento setup:upgrade
 3. php bin/magento setup:di:compile
 4. php bin/magento setup:static-content:deploy

create events.xml in the path:- app/code/Tw/Cart/etc/frontend/events.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="checkout_cart_product_add_after">
        <observer name="Tw_Cart_Addtocart_Customprice" instance="TW\Cart\Observer\CustomPrice" />
    </event>
</config>

create CustomPrice.php in the path: app/code/Tw/Cart/Observer/ & keep the below code

<?php
    namespace TW\Cart\Observer;
    use Magento\Framework\Event\ObserverInterface;
    use Magento\Framework\App\RequestInterface;
    use Psr\Log\LoggerInterface;
    class CustomPrice implements ObserverInterface
    {
    
    public function execute(\Magento\Framework\Event\Observer $observer) 
    {
            $writer = new \Zend\Log\Writer\Stream(BP.'/var/log/stackexchange.log');
            $logger = new \Zend\Log\Logger();
            $logger->addWriter($writer);
            /* Code here */
            $quote_item = $observer->getEvent()->getQuoteItem();
            $price = 400; //set your price here
            $quote_item->setCustomPrice($price);
            $quote_item->setOriginalCustomPrice($price);
            $quote_item->getProduct()->setIsSuperMode(true);
            $logger->info("success !!!!");
    }

}