I wanted to change the name and image of the header to the username of the people who are currently logged in but no idea how to do it I'm new to android studio and don't know how this works, i also tried to find the @string/nav_header_subtitle and i can't find it anywhere can anyone help me?
Edit:
This is the main activity
    public class MainActivity extends AppCompatActivity
    implements NavigationView.OnNavigationItemSelectedListener();
    NavigationView  navigationView = (NavigationView) findViewById(R.id.navigation_view);
    View header = navigationView.getHeaderView(0);
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
                this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.addDrawerListener(toggle);
        toggle.syncState();
        NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
        navigationView.setNavigationItemSelectedListener(this);
    }
    @Override
    public void onBackPressed() {
        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        if (drawer.isDrawerOpen(GravityCompat.START)) {
            drawer.closeDrawer(GravityCompat.START);
        } else {
            super.onBackPressed();
        }
    }
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }
    @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);
    }
    @SuppressWarnings("StatementWithEmptyBody")
    @Override
    public boolean onNavigationItemSelected(MenuItem item) {
        int id = item.getItemId();
        if (id == R.id.nav_profile) {
        } else if (id == R.id.nav_balance) {
        } else if (id == R.id.nav_setting) {
        } else if (id == R.id.nav_help) {
        } else if (id == R.id.nav_logout) {
            final ProgressDialog dlg = new ProgressDialog(MainActivity.this);
            ParseUser.logOut();
            moveTaskToBack(true);
            android.os.Process.killProcess(android.os.Process.myPid());
            System.exit(1);
        }
        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        drawer.closeDrawer(GravityCompat.START);
        return true;
      }
   }
Where should I put the code in there?
Edit: I did the codes below and this showed up
Edit: Header:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="@dimen/nav_header_height"
android:background="@drawable/side_nav_bar"
android:gravity="bottom"
android:orientation="vertical"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingBottom="@dimen/activity_vertical_margin"
android:theme="@style/ThemeOverlay.AppCompat.Dark">
<ImageView
    android:id="@+id/imageView"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:contentDescription="@string/nav_header_desc"
    android:paddingTop="@dimen/nav_header_vertical_spacing"
    app:srcCompat="@mipmap/ic_launcher_round" />
<TextView
    android:id="@+id/txt_namedisplay"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:paddingTop="@dimen/nav_header_vertical_spacing"
    android:text="@string/nav_header_title"
    android:textAppearance="@style/TextAppearance.AppCompat.Body2" />
<TextView
    android:id="@+id/textView"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/nav_header_subtitle" />
 
     
     
     
    