Customise 'add to cart' text on single product pages. Replace 'my button text' with whatever you want it to say.
add_filter( 'add_to_cart_text', 'woo_custom_cart_button_text' ); // < 2.1
add_filter( 'woocommerce_product_single_add_to_cart_text', 'woo_custom_cart_button_text' ); // 2.1 +
function woo_custom_cart_button_text() {
return __( 'My Button Text', 'woocommerce' );
}