The following is a very nice solution to replace a single character at a known position in a String, it uses the prototype functionality to add a method replaceCharAt to the String class:

  <script type=text/javascript>

    String.prototype.replaceCharAt=replaceCharAt;
 
    function replaceCharAt(character, position) {               
      var result = new RegExp('^(.{'+ --position +'}).(.*)$',''); 
      return this.replace(result,'$1'+character+'$2');
    }
  </script>

You can use this new function in the following way:

  <script type=text/javascript>

    var text = "This is some text";
    alert(text.replaceCharAt('H', 4));

  </script>