Usually what you need to do is look at the footer.php file to find what you're looking for. In most cases, the code is right there. In some cases, like Catch Everest, you'll have to dig a little deeper because a function is called from footer.php. For Catch Everest, that function is located in the /inc/catcheverest-functions.php file, so if you want to edit that file, you'll need to make a copy of it from the parent theme over to your child theme and change it there.
However, I always find it easier to just add some custom CSS to hide that information. If you use a web debugging tool like Firebug or Chrome Developer Tools, you'll see that in Catch Everest's case, there's a DIV with a class of powered that holds that information. So all you would need to do to hide that info is add this to Catch Everest's Custom CSS option (Appearance > Theme Options > Custom CSS):
footer .powered {
display: none;
}You can also add the CSS to the end of your child theme's style.css if the theme you are using doesn't have a Custom CSS option. Or, if the changes you're making don't require the creation of a child theme (i.e., just CSS changes), you can install a CSS plugin.