I have a iPad how can I update Safari. I read thI have to update Safari to use Whatspp using iPad.

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/

As to your question regarding Safari, this is an integral element of iOS/iPadOS - and cannot be updated independently of the OS. If your iPad has been updated to the most recent version supported by your iPad (the model and installed version of iOS have not been declared), then Safari is as up-to-date as is possible.

However, some websites incorrectly identify your iOS/iPadOS Safari User Agent when used in one or other of its browsing modes - of which their are two - these being Desktop and Mobile.

It’s easy to switch between modes if a website isn’t functioning correctly. From the desired web-page, touch the aA icon at the left of the Address Bar; this will reveal a menu. From the menu select Website Settings, then select (or deselect) Request Desktop Website, then tap Done. Now refresh the webpage.

The chosen setting will be saved for the selected website - and can be simply reversed if needed.

There are no specific updates for Safari. If it is updated then it would be part of an update of iOS, so if your iOS version is up to date then so is Safari.