How should I make changes in existing function of function.pnp?

Hello. I’m trying to make my first blog. And I will be really thankful for help.

2012-theme seems good for me with some changes that i put in child-theme. Blog is in Russian. And Open Sans font does not work by default. I need to switch it to cyrillic.

Looks like I can do it in a function twentytwelve_scripts_styles() in functions.php of original 2012. I need to change $subsets = ‘latin,latin-ext’; to $subsets = ‘latin,latin-ext,cyrillic,cyrillic-ext’; in function body. And it’s gonna work.

Am I looking in the right way?

But I don’t want to change original functions.php. I want to make changes in child-theme. How can I do it here?

I tryid to wrore:

if (!function_exists('twentytwelve_scripts_styles')) {
    function twentytwelve_scripts_styles() {
        //original function code
        $subsets = 'latin,latin-ext,cyrillic,cyrillic-ext';
        //original function code
    }
}

but get and error:
Cannot redeclare twentytwelve_scripts_styles() (previously declared in …functions.php