Add this code to your child theme’s functions.php file or via a plugin that allows custom functions to be added, such as the Dessky Snippets plugin. Please don’t add custom code directly to your parent theme’s functions.php file as this will be overwritten completely when you update the theme.

// Change add to cart text on single product page
add_filter( 'woocommerce_product_single_add_to_cart_text', 'woocommerce_add_to_cart_button_text_single' ); 
function woocommerce_add_to_cart_button_text_single() {
    return __( 'Add to Cart Button Text', 'woocommerce' ); 
}

// Change add to cart text on product archives page
add_filter( 'woocommerce_product_add_to_cart_text', 'woocommerce_add_to_cart_button_text_archives' );  
function woocommerce_add_to_cart_button_text_archives() {
    return __( 'Add to Cart Button Text', 'woocommerce' );
}