Using jQuery's .html() to change another webpage on my WordPress website

Using jQuery's .html() to change another webpage on my WordPress website

I can easily use .html() to change the html on the webpage I'm on, but I'm
trying to use it from my main .js page to modify any webpage on my
WordPress Genesis (Associate theme) website. I've put the url and id# in
$("URL#idname").html("my html"); and even tried
$("#idname").load("URL").html("my html"); to no avail. I've read other
people trying $.ajax() but can't make that work, either. In fact, my .js
page is using jQuery as it is, so I can't understand why I can't affect
any pages on my site?