How to remove the hamburger icon and put the back icon [Android]

How can I remove the Hamburger icon from my NavigationView when I go to another fragment and then change it to a "back" icon?

NavigationActivity.kt:

package com.example.gonzalo.proyecto_android_2018.activities

import android.support.v4.app.Fragment
import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.support.design.widget.NavigationView
import android.support.v4.view.GravityCompat
import android.support.v7.app.ActionBarDrawerToggle
import android.support.v7.widget.Toolbar
import android.view.MenuItem
import com.example.gonzalo.proyecto_android_2018.R
import com.example.gonzalo.proyecto_android_2018.fragments.*
import kotlinx.android.synthetic.main.activity_navigation.*

class NavigationActivity : AppCompatActivity(), NavigationView.OnNavigationItemSelectedListener{

lateinit var tb:Toolbar

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_navigation)

    //SETUP TOOLBAR
    tb = toolbar as Toolbar
    //tb.title = "LIGHT BUS"

    //SETUP NAVIGATIONVIEW
    setupNavigation()

    setupFragment(HomeFragment())

    navigation.menu.getItem(0).isChecked = true
}

private fun setupNavigation() {

    //NO EXISTE LA CLASE Toogle, LO DIGO PARA NO CONFUNDIRSE CON LA VARIABLE "toogle"
    val toogle = ActionBarDrawerToggle(this, drawer, tb, R.string.drawerAbierto, R.string.drawerCerrado)

    toogle.isDrawerIndicatorEnabled = true //ACA LE DIGO QUE ME APAREZCA EL SIMBOLO DEL SANDWICHITO PARA PODER ABRIR Y CERRAR EL NAVIGATIONVIEW

    drawer.addDrawerListener(toogle)
    toogle.syncState() //ACA LE DIGO QUE SINCRONIZE EL ESTADO ABIERTO O CERRADO. EJEMPLO, DEBE CERRAR CUANDO ESTE ABIERTO Y DEBE ABRIRSE CUANDO ESTE CERRADO

    navigation.setNavigationItemSelectedListener(this) //ACA LE DIGO A ESTE NAVIGATION(navigation) QUE VA A ESTAR YA AÑADIDO DENTRO DEL NAVIGATIONACTIVITY
}

private fun setupFragment(fragment: Fragment){       //Reemplaza el "R.id.frameLayout" por el id del "fragment" que se va a colocar en el "activity_navigation.xml"
    supportFragmentManager.beginTransaction().replace(R.id.frameLayout, fragment).commit()
}

override fun onNavigationItemSelected(item: MenuItem): Boolean {
    when(item.itemId){
        R.id.home -> setupFragment(HomeFragment())
        R.id.mi_cuenta -> setupFragment(MiCuentaFragment())
        R.id.pagos -> setupFragment(PagosFragment())
        R.id.promociones -> setupFragment(PromocionesFragment())
        R.id.regalo_amigo -> setupFragment(RegaloAmigoFragment())
        R.id.ayuda -> setupFragment(AyudaFragment())
    }
    drawer.closeDrawer(GravityCompat.START) //ACA LE DIGO, DESPUES DE QUE HAYAS CAMBIADO DE FRAGMENT, CIERRA EL DRAWER
    return true
}

override fun onBackPressed() {
    if(drawer.isDrawerOpen(GravityCompat.START)){
        drawer.closeDrawer(GravityCompat.START)
    }

    else{
        super.onBackPressed()
    }
}

}

activity_navigation.xml:

<?xml version="1.0" encoding="utf-8"?>

<android.support.v4.widget.DrawerLayout
android:id="@+id/drawer"
android:fitsSystemWindows="true"
android:layout_width="match_parent"
android:layout_height="match_parent"
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
>

<android.support.constraint.ConstraintLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <include
        android:id="@+id/toolbar"
        layout="@layout/toolbar"/>

    <FrameLayout
        android:id="@+id/frameLayout"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_marginTop="?attr/actionBarSize">

    </FrameLayout>

</android.support.constraint.ConstraintLayout>

<android.support.design.widget.NavigationView
    android:id="@+id/navigation"
    app:itemTextColor="#009688"
    app:menu="@menu/main_menu"
    app:headerLayout="@layout/nav_header"
    android:fitsSystemWindows="true"
    android:layout_gravity = "start"
    android:layout_width="wrap_content"
    android:layout_height="match_parent"/>

</android.support.v4.widget.DrawerLayout>