How to place an aspx page inside update panel of another aspx page

Is it possible to place an aspx page inside the update panel of another aspx page? If so, how can I do that?

you could try out UFrame (Update Panel + IFrame), have a look at http://uframe.codeplex.com/

You can put the UFrame in your main.aspx, and specify the src property to your existing aspx page, as below:

In mainpage.aspx:

<div class="UFrame" id="UFrame1" src="ExistingPage.aspx" >
  <p>This should get replaced with content from Existingpage.aspx</p>
</div>