Add the following code to the bottom of custom-functions.php file. You can change the Available! and Sold Out text to be whatever you like:
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;
}