There are multiple ways to do these.
I) Get main parent directly.
for exa. i want to replace my child page to iframe then
var link = '<%=Page.ResolveUrl("~/Home/SubscribeReport")%>';
top.location.replace(link);
here top.location gets parent directly.
II) get parent one by one,
var element = $('.iframe:visible', window.parent.document);
here if you have more then one iframe, then specify active or visible one.
you also can do like these for getting further parents,
var masterParent = element.parent().parent().parent()
III) get parent by Identifier.
var myWindow = window.top.$("#Identifier")