I assume you want to download the program to your PC first and then install it onto your device.
Go to Google play and search for Whatsapp.
Visit the Whatsapp page and copy the browser URL
Go to http://apps.evozi.com/apk-downloader/
then paste the URL and click on download
Download the apk to your computer and transfer it to your device.
Just make sure you have selected install from untrusted sources on your device before you attempt to install the apk file.
Install the file.