If you mean the actual content inside the iframe then you can't change that content as it doesn't belong to you
The only way to change the style of the html in an iframe is by adjusting the css in the page that the html belongs to. An iframe is basically a window that lets you see outside your site. You can't change the view by changing the window.
If the iframe content is hosted on your own site and you have access to the css then you would need to design the page in a responsive manner so that it fits the viewport accordingly.