Theme Not Responsive in Ipad

I am cuurently using vanguard theme for my wordpress website. The theme is properly responsive in all devices except using ipad. When I use ipad in portrait mode, the titles in the menu bar overlap with the logo. So, i want to change the settings such that when using ipad in portrait mode, the website opens in the way it opens in mobile devices. So, how and in which file can I change these settings ???