Various websites say safari needs to be upgraded. However I am using most up to date iOS on my iPad. What wrong?

What iOS version is running on your iPad exactly? Settings ➜ General ➜ About ➜ Version

What model of iPad do you have? click here ➜ Identify your iPad model - Apple Support

While it may be the most up to date iOS version for your iPad, it may not be the most up to date version available in general, which means your Safari version may now be considered a bit too old and outdated by some websites.

Safari 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.

You can check to see how your browser is identified, in each of its modes, using this website:

https://www.whatismybrowser.com/

Since you have provided no details, its impossible to know wether there is something that can be done or not.

Provide the model of iPad the iOS version it is running, and more specific advice may be able to be given.

There may be an alternative browser you could use that is more up to date and those websites can accept.