If you ftp into the site, your template code is going to be under the
templates folder. You can find which specific template is being used by going in the admin to
Extensions > Templates > Templates. There should be a subfolder with an appropriate name in the
templates folder. This contains your template files.
If your site is responsive (ie the mobile version is generated using css media queries rather than redirecting to a completely different template), then you should be able to find and comment out any media queries in the css files. CSS files are likely to be under a
css folder within this template.
Media queries look like
@media (max-width: 800px).
You could alternatively add/override a viewport meta tag in the
index.php file in order to to tell mobile devices to always display the site at the same width, which would be a quick fix if it is responsive.
If the site is adaptive (ie it loads a completely different template), then you may be best doing a case insensitive search for terms like "iphone" in the template files. If this finds nothing, go to
Extensions > Plugins in the admin and see if there is a plugin doing the redirect. Unlike Wordpress, Joomla does not do mobile detection out-of-the-box, and so these are the most likely places you will find someone has added a script to add bespoke adaptive elements.
You can find if your site is responsive or adaptive by reducing your browser window. If a small desktop window shows the mobile version, it is responsive.