To rename the WooCommerce "Add to Cart" button if the product is already added to the cart, you'll need to use some custom code. This involves adding a code snippet to your theme's functions.php file or by using a custom plugin. Below are the steps to achieve this:

1. Backup Your Website: Before making any changes, it's always a good idea to create a backup of your website, including your database and files.

2. Access Theme Files or Use a Child Theme: You can access your theme's functions.php file through your WordPress admin panel by navigating to "Appearance" > "Theme Editor." Alternatively, it's recommended to use a child theme if you're not already using one. This helps prevent your customizations from being overwritten during theme updates.

3. Add Custom PHP Code: Add the following code to your theme's functions.php file or to a custom plugin (if you prefer using a plugin for this purpose):  

function custom_rename_add_to_cart_button( $button_text, $product ) { 
// Get the product ID 
$product_id = $product->get_id(); 
// Check if the product is already in the cart 
if ( WC()->cart->find_product_in_cart( WC()->cart->generate_cart_id( $product_id ) ) ) { 
    $button_text = __('Product Already in Cart', 'woocommerce'); } return $button_text; 
} 
add_filter( 'woocommerce_product_single_add_to_cart_text', 'custom_rename_add_to_cart_button', 10, 2 ); 
add_filter( 'woocommerce_product_add_to_cart_text', 'custom_rename_add_to_cart_button', 10, 2 ); 

4. Save Changes: After adding the code snippet, save the changes to your functions.php file or your custom plugin.

5. Clear Cache: If you're using any caching plugins or server-side caching mechanisms, clear your cache to ensure that the changes take effect.

6. Test: Visit a product page on your WooCommerce store where the "Add to Cart" button is displayed. If the product is already in the cart, the button text should now display "Product Already in Cart" instead of the default "Add to Cart."

Remember that modifying code directly can have implications for your website's functionality and future updates. Always make sure you have backups and consider testing this in a development environment before applying it to your live site.