.

 Just replace FUNCTION_NAME with unique function name and place in functions.php file.

add_filter( 'gettext', 'replace_FUNCTION_NAME_text' );

function replace_FUNCTION_NAME_text( $text ){

    if( $text === 'Any category' ) {
        $text = 'Any Subject';
    }

    return $text;
}

ANOTHER FUNCTION BELOW

add_filter('gettext', 'translate_text'); 
add_filter('ngettext', 'translate_text');

function translate_text($translated) { 
$translated = str_ireplace('each month', 'per month', $translated);
$translated = str_ireplace('on the', '', $translated); 
$translated = str_ireplace('of', '', $translated); 
return $translated; 
}