how to modify layout in android studio

To prevent the Toolbar from being displayed, you must define the following theme within the Styles.xml file:

<style name="AppTheme.NoActionBar">
    <item name="windowNoTitle">true</item>
    <item name="windowActionBar">false</item>
    <item name="android:windowFullscreen">true</item>
    <item name="android:windowContentOverlay">@null</item>
</style>

and in the definition of your Activity you add the theme, for example:

  <activity
            android:name=".MainActivity"
            android:theme="@style/AppTheme.NoActionBar">

This way you will have your application without the Toolbar (ActionBar).

You should modify certain parameters in the layout of that activity. You must find the element that represents toolbar and change the value of its property height to match-parent

You can change the theme for your activity (without changing the general theme) in the following way:

<activity android:name=".MiActividadSinAppBar"
      android:label="@string/mi_app"
      android:theme="@android:style/Theme.Black.NoTitleBar">

For your MainActivity then:

<activity android:name=".MainActivity"
      android:label="@string/mi_app"
      android:theme="@android:style/Theme.Black.NoTitleBar">

To define a style with its own theme without AppBar, you place the following in your values/styles.xml :

        false         true     

and in your layout:

        android:theme="@style/AppTheme.NoActionBar"