MainActivity.this.runOnUiThread(new Runnable() {
    public void run() {
        Log.d("UI thread", "I am the UI thread");
    }
});

timer = new CountDownTimer(10* 1000, 1000) { //каждую секунду в течении 10 секунд

            public void onTick(long millisUntilFinished) {
               String text = GetNewText();
               TextView tv.setText();
            }

            public void onFinish() {
               super.onFinish(result);
            }
 }.start();