Woocommerce empty cart before add new product to cart.

wc-one-product-add-to-cart.php

<?php
/**
 * Empty cart. New product will be added. Works with WooCommerce 3.0+
 */
add_filter( 'woocommerce_add_to_cart_validation', 'custom_only_one_in_cart', 99, 2 );
  
function custom_only_one_in_cart( $passed, $added_product_id ) {
	wc_empty_cart();
	return $passed;
}