Javascript writting the value of innerHtml using document.write

<div id="para">
    <p>Lorem</p>
</div>

    var test= document.getElementById("para").innerHtml("<p>ipsum</p>");
    var test2 =document.getElementById("para");
    document.write(test2);

I want to write the value of test2 variable or what is inside innerHTML.But its not working,can you give me reason why that is happening.