Edit in JSFiddle

function change(){
    
   var myNewTitle = document.getElementById('myTextField').value;
   if( myNewTitle.length==0 ){
       alert('Write Some real Text please.');
   return;
   }
   
   var title = document.getElementById('title');
   title.innerHTML = myNewTitle;
    
}

<h1 id="title">Javascript example no.2</h1>
<input type="text" id="myTextField"/>
<input type="submit" id="byBtn" value="Change" onclick="change()"/>