function fix_country_name_mailchimp( $user_meta, $user_id ) {

    foreach( $user_meta as $key => $value ) {

        if( $value == 'United States' ) {
            $user_meta[$key] == 'United States of America';
        }

    }

    return $user_meta;

}

add_filter( 'wpf_user_update', 'fix_country_name_mailchimp', 10, 2 );