My whatsapp is not working since I failed to upgrade-now how do I upgrade it?

.. hi
I have started using smart phone very recently and I have very basic idea about android, I primarily use whatsapp and only for calling. My Whatsapp kept prompting asking me to update, but I had not updated as i did not have internet connectivity at that time. Now my Whatsapp doesn't work at all. Should i uninstall and reinstall?? Since i am new could you please tell me where to navigate to get the Marketplace is to download application the new Whatsapp.

Thank you.

Its easy to upgrade, just open Play store, you can find it in app list and then search "whatsapp" , it will show you whatsapp, tap on it. If it is already installed, then you will see option to upgrade otherwise install.

You can also take following route :
Play store --> My apps --> Upgrade all (or any desired installed app).