Empty Woocommerce Cart before adding new item | only 1 product add to cart woocommerce, Code goes in function.php file of your active child theme (or theme)

functions.php

<?php

// before addto cart, only allow 1 item in a cart
add_filter( 'woocommerce_add_to_cart_validation', 'woo_custom_add_to_cart_before' );
function woo_custom_add_to_cart_before( $cart_item_data ) {
    global $woocommerce;
    $woocommerce->cart->empty_cart();
    // Do nothing with the data and return
    return true;
}

?>