res/xml/filepaths.xml

<paths xmlns:android="http://schemas.android.com/apk/res/android">
    <external-path name="external" path=""/>
</paths>

<provider
    android:name="android.support.v4.content.FileProvider"
    android:authorities="com.yzj.application.fileprovider"
    android:exported="false"
    android:grantUriPermissions="true">
    <meta-data
        android:name="android.support.FILE_PROVIDER_PATHS"
        android:resource="@xml/filepaths"/>
</provider>

Intent intent = new Intent(Intent.ACTION_VIEW);
File apkFile = new File(apkPath);
intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
    intent.setFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION);
    Uri uri = FileProvider.getUriForFile( MainActivity.this,getPackageName() + ".fileprovider", apkFile);
    intent.setDataAndType(uri, "application/vnd.android.package-archive");
} else {
    intent.setDataAndType(Uri.fromFile(apkFile), "application/vnd.android.package-archive");
}
startActivity(intent);