How to create a back button in quiz app?

 public class MainActivity extends AppCompatActivity {
             List<Questions> quesList;
             int score=0;
             int qid=0;
            Questions currentQ;
            TextView tv;
            RadioButton rb1,rb2,rb3,rb4;
            ImageButton next,back;
            Questions cur;
            @Override
            protected void onCreate(Bundle savedInstanceState) {
                super.onCreate(savedInstanceState);
                setContentView(R.layout.activity_main);
                DbHelper db=new DbHelper(this);
                final RadioGroup grp=(RadioGroup) findViewById(R.id.radiogroup1);
                quesList=db.getAllQuestions();
                if(quesList!= null && quesList.size() !=0) {
                    currentQ=quesList.get(qid);
                }
                tv=(TextView) findViewById(R.id.tv1);
                rb1=(RadioButton) findViewById(R.id.radio1);
                rb2=(RadioButton) findViewById(R.id.radio2);
                rb3=(RadioButton) findViewById(R.id.radio3);
                rb4=(RadioButton) findViewById(R.id.radio4);
                next=(ImageButton) findViewById(R.id.forward);
                back=(ImageButton) findViewById(R.id.backward);
                setQuestionView();

      //code for next button..      

    next.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {


                    RadioButton answer=(RadioButton) findViewById(grp.getCheckedRadioButtonId());

                    if(currentQ.getAnswer().equals(answer.getText()))
                    {
                        score++;
                        Log.d("score", "Your score" + score);
                    }
                    if(qid<5){
                        currentQ=quesList.get(qid);
                        setQuestionView();

                        qid++;
                        grp.clearCheck();
                    }else{
                        Intent intent = new Intent(MainActivity.this, ResultActivity.class);
                        Bundle b = new Bundle();
                        b.putInt("score", score); //Your score
                        intent.putExtras(b); //Put your score to your next Intent
                        startActivity(intent);
                        finish();
                    }
                }
            });
            back.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    //How to go to previous question?Please help me.
                }
            });
        }
        @Override
        public boolean onCreateOptionsMenu(Menu menu) {
            // Inflate the menu; this adds items to the action bar if it is present.
            getMenuInflater().inflate(R.menu.menu_main, menu);
            return true;
        }
        private void setQuestionView()
        {
            tv.setText(currentQ.getQuestion());
            rb1.setText(currentQ.getOption1());
            rb2.setText(currentQ.getOption2());
            rb3.setText(currentQ.getOption3());
            rb4.setText(currentQ.getOption4());

        }

        @Override
        public boolean onOptionsItemSelected(MenuItem item) {
            // Handle action bar item clicks here. The action bar will
            // automatically handle clicks on the Home/Up button, so long
            // as you specify a parent activity in AndroidManifest.xml.
            int id = item.getItemId();

            //noinspection SimplifiableIfStatement
            if (id == R.id.action_settings) {
                return true;
            }
            return super.onOptionsItemSelected(item);
        }

    }

What I want to create is back button in my quiz app, the questions come from database. I want button to go to previous question without messing up.I am new to android can any one help me to solve the problem..