Steps to  stop updating WhatsApp:

• Open the Settings app on your iPhone or iPad. It's a home screen app with an icon that looks like a series of grey cogs.
• Instead of being on the desktop, the Settings icon could be in the 'Utilities' folder.
• Tap iTunes & App Store at the bottom of the page. It's in the fourth set of choices.
• The Updates button should be turned off. The slider will change colour to white. WhatsApp's automatic updates have been deactivated. Unless you explicitly update the app, iOS will not update it.
• Go to the Google Play Store on your Android device. On the top-left, tap the hamburger icon (three horizontal lines). Go to the Settings tab. Select Auto-update apps from the drop-down menu. Select Do not auto-update apps to turn off automatic app updates.

Read More: How can I ignore the WhatsApp upgrade?