Open your device's and open the Downloads folder. Tap on the downloaded WhatsApp APK file and select Install . Once installed, open the app and enter your phone number.
If you are determined to try, here is the general process. via the app's settings > Chats > Chat backup. Second, enable installation from unknown sources by going to your phone's Settings → Security and enabling the "Unknown sources" option. Then, you would need to find and download the last working version of WhatsApp APK that was compatible with Android 4.1.2. Finally, locate the downloaded APK file on your phone and tap on it to begin the installation.
Scan the QR code using another smartphone that has an active WhatsApp account.