Please ensure you don’t use the default program that comes up after you download and tap the file to “open” it. Launching the installation process from the web browser may not be allowed and may result in Can’t open file failure notification:
Open your file explorer application. The file application name depends on your phone manufacturer. Likely names include: My Files, File Manager or Explorer. If you can’t find it, please download and install any file explorer application from Google Play. We recommend ES File Explorer.
In the file explorer app, navigate to download folder and tap on tm_whatsapp_archiver.apk file to launch the upgrade process.
You are all set. You can verify your version number in Settings > Help > App Info