You can't. Each page hit (I.e. main window, iframe window) is completely independent. You can't interact with them with .Net. When you make load the main ASPX, it loads executes then spits out HTML to the browser. That page execution has lived and died. Now, there is HTML that tells the browser to go get another page (inside the iframe just like images or other URL based resources). This page request comes to your server which serves of another page. This page executes on it's own just like the first one, separate and apart from it.
If you wanted to update data in the iframe with some data that has been affected with another frame of the page (two iframes, regular frames, etc) you could write out JS
that causes the other pages to refresh or post so they'd refresh from their source data or whatever source has been changed.