How to install what’s up on this iPad

Presumably you refer to WhatsApp…

The WhatApp developers have not released an App compatible with iPad - the App being currently limited to iPhone.

Whilst the App is not available for iPad - you should be able to access the service using a web browser:

https://web.whatsapp.com/

Assuming you mean "WhatsApp", you cannot, there is no version of the WhatsApp app for iPad.

You can use the website at https://web.whatsapp.com and connect it to your phone.

If you don't see the QR code to connect it, tap on the aA icon on the search bar and tap on "Request Desktop site".