You may have to uninstall this app then install a new version. (Typical Windows)
I have had a problem with hotspot and made a soft reset. My phone is Lumia 430 running Windows 8.1.
After the reset I tried to login in Whatsapp, but the date was wrong and I got a message to change it. After I changed it, I got a message that I must update whatsapp. I updated.
However, when I try to run it again, it still showed me that I need to update. Now I'm in the loop. The date is correct, I have the latest version of Whatsapp, but I still get a message that I must update. That message doesn't allow me to enter into Whatsapp. Otherwise I would just backup the chat history and re-install it.