Cancel and register new alarm

I'm trying to use the alarmmanager for my android app.

if the username change, I wanna cancel the current alarm and set a new one for the current username

The problem is that am.cancel(sender); doesn't cancel the alarm

Also, I don't see the new alarm for the new username.

Here's my code

private AlarmManager am;
private PendingIntent sender;
public void registerAlarm() {

        //Cancel the current alarm
        if(am != null){
            am.cancel(sender);
        }

        Intent i = new Intent(this, AlarmReceiver.class);
        i.putExtra("username", username);
        sender = PendingIntent.getBroadcast(this, 0, i, 0);

        // We want the alarm to go off 3 seconds from now.
        long firstTime = SystemClock.elapsedRealtime();
        firstTime += 3 * 1000;//start 3 seconds after first register.

        // Schedule the alarm!
        am = (AlarmManager) this
                .getSystemService(ALARM_SERVICE);
        am.setRepeating(AlarmManager.ELAPSED_REALTIME_WAKEUP, firstTime,
                60000, sender);//10min interval
    }
}

Thank you

EDIT

I also tried this code without success

 public void SetAlarm(Context context, String username)
    {
        //Cancel the current alarms
        CancelAlarm(context);

        AlarmManager am=(AlarmManager)context.getSystemService(Context.ALARM_SERVICE);
        Intent i = new Intent(context, AlarmReceiver.class);
        i.putExtra("username",username);
        PendingIntent pi = PendingIntent.getBroadcast(context, 0, i, 0);

        // We want the alarm to go off 3 seconds from now.
        long firstTime = SystemClock.elapsedRealtime();
        firstTime += 3 * 1000;//start 3 seconds after first register.

        // Schedule the alarm!
        am.setRepeating(AlarmManager.ELAPSED_REALTIME_WAKEUP, firstTime,
                60000, pi);//1min interval
    }

    public void CancelAlarm(Context context)
    {
        Intent intent = new Intent(context, AlarmReceiver.class);
        PendingIntent sender = PendingIntent.getBroadcast(context, 0, intent, 0);
        AlarmManager alarmManager = (AlarmManager) context.getSystemService(Context.ALARM_SERVICE);
        alarmManager.cancel(sender);
    }