// To change add to cart text on single product page
add_filter( 'woocommerce_product_single_add_to_cart_text', 'woocommerce_custom_single_add_to_cart_text' ); 
function woocommerce_custom_single_add_to_cart_text() {
    return __( 'Buy Now', 'woocommerce' ); 
}
// To change add to cart text on product archives(Collection) page
add_filter( 'woocommerce_product_add_to_cart_text', 'woocommerce_custom_product_add_to_cart_text' );  
function woocommerce_custom_product_add_to_cart_text() {
    return __( 'Buy Now', 'woocommerce' );
}

function so_validate_add_cart_item( $passed, $product_id, $quantity, $variation_id = '', $variations= '' ) {

    // do your validation, if not met switch $passed to false
    if ( 1 != 2 ){
        $passed = false;
        wc_add_notice( __( 'You can not do that', 'textdomain' ), 'error' );
    }
    return $passed;

}
add_filter( 'woocommerce_add_to_cart_validation', 'so_validate_add_cart_item', 10, 5 );

add_action( 'woocommerce_before_add_to_cart_button', 'misha_before_add_to_cart_btn' );

function misha_before_add_to_cart_btn(){
	echo 'Some custom text here';
}

// To change add to cart text on single product page
add_filter( 'woocommerce_product_single_add_to_cart_text', 'woocommerce_custom_single_add_to_cart_text' ); 
function woocommerce_custom_single_add_to_cart_text() {
    return __( 'Buy Now', 'woocommerce' ); 
}
// To change add to cart text on product archives(Collection) page
add_filter( 'woocommerce_product_add_to_cart_text', 'woocommerce_custom_product_add_to_cart_text' );  
function woocommerce_custom_product_add_to_cart_text() {
    return __( 'Buy Now', 'woocommerce' );
}