Solution

<input type="text" id="input" size="30">
<input type="text" id="output" size="30">
<input type="button" id="btn" value="Update">

<script>
function update() {
  console.log('update');
  var input = document.getElementById('input').value;
  console.log(input);
  document.getElementById('output').value = input;
}

document.getElementById('btn').addEventListener('click', update);
</script>