import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    public void Facebook(View v){
        Button facebook_btn = (Button) findViewById(R.id.facebook_id);
        facebook_btn.setBackgroundResource(R.drawable.youtube);

    }
}

<?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"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.formation.designbutton.MainActivity"
    android:orientation="horizontal"
    android:gravity="center">

    <Button
        android:id="@+id/facebook_id"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="@drawable/facebook"
        android:layout_margin="10px"
        android:onClick="Facebook"/>

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="@drawable/youtube"
        android:layout_margin="10px"/>

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="@drawable/twitter"
        android:layout_margin="10px"/>

</LinearLayout>