here is the code
button1 = (Button) findViewById(R.id.button1);
button1.setOnClickListener(new OnClickListener()
{
public void onClick(View v1)
{
Intent promptInstall = new Intent(Intent.ACTION_VIEW)
.setData(Uri.parse("file:///path/to/your.apk"))
.setType("application/vnd.android.package-archive");
startActivity(promptInstall);
}
}