Solution:
You can’t manipulate what is inside an iframe but, if the content You want to hide is an horizontal navigation section at top of the page, you could always wrapped the iframe inside a div with overflow:hidden and then, move the content up with top propertie.
Like this:
div {
overflow:hidden;
}
iframe {
position:relative;
top:-120px;
border:0;
width:100%;
}<h2>HTML Iframes</h2>
<p>I want to remove the navigation bar:</p>
<div>
<iframe id="" class=" " scrolling="no" style=" height: 1500px; overflow: hidden;" src="https://www.wikipedia.org/" title="Iframe"></iframe>
</div>