update data in sqlite database android

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hi all..
i m very new to android development...i have created one app which works well for inserting into and selecting data from database...
but i don't know how to update data in database...
here is code i have tried...but get NullPointerException...

if (v.getId()==R.id.btnUpdate)
          {
            Cursor value = null ;


            dbAdaptert.open();
            String name=value.getString(value.getColumnIndex("name"));

             String pass=etpass.getText().toString();
             String uname=etuname.getText().toString();

            long update=dbAdaptert.updateTest(name, uname, pass);

            if(update>=0)
            {
                Toast.makeText(getApplicationContext(), "Data updated",Toast.LENGTH_LONG).show();

            }
            else
            {
                Toast.makeText(getApplicationContext(), "Data not updated",Toast.LENGTH_LONG).show();

            }
            dbAdaptert.close();



          }