Untitled

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Document</title>
</head>
<body>
  <a id="" class="btn" href="#"><button>Green</button></a><br><br>
  <a id="changeId" href="#"><button>Change Id!</button></a>
</body>
</html>

$(document).ready(function() {

  $("a#green").on("click", function(){
    $("body").css("background-color","green");
  });
  $("a#changeId").on("click", function(){
    $("a.btn").attr("id","green");
  });

});