This is a perfect example of when it is appropriate to use a ternary expression in PHP. It will evaluate a condition and use one value for true and another for false. This is the line of code you posted, after the ? is when the expression evaluates to true and after the : is when it evaluates to false. So you can alter it by also checking if the value is one, then output either yes or no like this:
(isset($_POST["UserNewsletter"]) && $_POST["UserNewsletter"] == '1')?"yes":"no");