@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_admin);
    ListView lv= (ListView) findViewById(R.id.lv);
    registerForContextMenu(lv);
    MyAdapter adapter=new MyAdapter();
    adapter.notifyDataSetChanged();
    adapter.context= this;
    adapter.list=GlobalVar.users;
    lv.setAdapter(adapter);

}


@Override
public boolean onContextItemSelected(MenuItem item) {
   if (item.getItemId()==R.id.dob)
   {
       Intent intent= new Intent(AdminActivity.this,RegActivity.class);
       startActivity(intent);
   }
    return super.onContextItemSelected(item);
}

@Override
public void onCreateContextMenu(ContextMenu menu, View v, ContextMenu.ContextMenuInfo menuInfo) {
    super.onCreateContextMenu(menu, v, menuInfo);
    MenuInflater inflater= getMenuInflater();
    inflater.inflate(R.menu.menu,menu);
}

class  MyAdapter extends BaseAdapter
{
    ArrayList<User> list;
    Context context;

    @Override
    public int getCount() {
        return list.size();
    }

    @Override
    public User getItem(int position) {
        return list.get(position);
    }

    @Override
    public long getItemId(int position) {
        return list.get(position).getId();
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        View con= convertView;
        if(con== null)
        {
            LayoutInflater inflater= (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            con= inflater.inflate(R.layout.list_item,parent,false);

        }
        TextView loginTxt= (TextView) con.findViewById(R.id.userloginTxt);
        TextView nameTxt= (TextView) con.findViewById(R.id.usernameTxt);
        TextView passwordTxt= (TextView) con.findViewById(R.id.userpasswordTxt);
        loginTxt.setText(list.get(position).getLogin());
        nameTxt.setText(list.get(position).getName());
        passwordTxt.setText(list.get(position).getPassword());
        return con;
    }
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_reg);
    final EditText nameReg= (EditText) findViewById(R.id.editReg);
    final EditText passReg= (EditText) findViewById(R.id.editReg2);
    final EditText passReg2= (EditText) findViewById(R.id.editReg3);
    final EditText loginReg=(EditText) findViewById(R.id.editReg4);
    Button btnReg= (Button) findViewById(R.id.btmReg);
    Button btnReg2=(Button) findViewById(R.id.btmReg2);
    btnReg2.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            finish();
        }
    });
    btnReg.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
          User user= new User();
            user.setLogin(loginReg.getText().toString());
            user.setPassword(passReg.getText().toString());
            user.setName(nameReg.getText().toString());
            if (SaveData(user))
            {
               finish();
            }
            else
            {
                Toast.makeText(RegActivity.this,"Ошибка", Toast.LENGTH_LONG).show();
            }
            }


    });
}
boolean SaveData(User user)
{
    DBHelper helper= new DBHelper(this);
    SQLiteDatabase db= helper.getWritableDatabase();
    ContentValues cv= new ContentValues();
    cv.put("login", user.getLogin());
    cv.put("password", user.getPassword());
    cv.put("name", user.getName());
    long rowId= db.insert("user",null,cv);
    if (rowId>0)
    {
        return true;
    }
    return false;
}

//Вам нужно будет ListView сделать static

public static void refreshAdapter(Context ctx){
   MyAdapter adapter = new MyAdapter();
   adapter.notifyDataSetChanged();
   adapter.context = ctx;
   adapter.list = GlobalVar.users;
   lv.setAdapter(adapter);
}

if (SaveData(user))
 {
  ВашПервыйАктивити.refreshAdapter(getApplicationContext());
  finish();
 }