Use the following code to change the text displayed on a WooCommerce product's page when it is In Stock or Out of Stock. Simply add the following code to the 'custom functions' area of your functions.php file:

ChangeWooCommerceStockText.php

<?php

/**
 * Change In Stock / Out of Stock Text
 */

add_filter( 'woocommerce_get_availability', 'wcs_custom_get_availability', 1, 2);
function wcs_custom_get_availability( $availability, $_product ) {
   
   	// Change In Stock Text
    if ( $_product->is_in_stock() ) {
        $availability['availability'] = __('Available!', 'woocommerce');
    }

    // Change Out of Stock Text
    if ( ! $_product->is_in_stock() ) {
    	$availability['availability'] = __('Sold Out', 'woocommerce');
    }

    return $availability;
}