First of all, download WhatsApp in your android phone from the link provided above. If you want to back up your data, restore it from your Whatsapp. Once the data is backed up, uninstall WhatsApp from your Android phone. Install the Whatsapp APK file on your Android phone that you have already downloaded from the above link.
After the installation process, register your mobile number with your WhatsApp and complete the verification process. After the verification, you are ready to enjoy the exclusive features of Whatsapp Plus.