how do i update my ipad that is old

Whether you are able to update your iPad to a newer version of iOS/iPadOS will be dictated by your iPad model - and the currently installed version of iOS. You can find this information here:

Settings > General > About

Here is a list of the highest version of iOS supported by the older models of iPad (for simplicity - “generations” are shortened to a number) that are restricted in their ability to be updated:

iPad1 - iOS 5.1.1

iPad2, iPad3 and iPad mini1 - iOS 9.3.5 (WiFi Only models), iOS 9.3.6 (WiFi & Cellular)

iPad4 - iOS 10.3.3 (WiFi Only), iOS 10.3.4 (WiFi & Cellular

iPad Air1, iPad mini2 and iPad mini3 - iOS 12.4.7

Newer models, not appearing in this list, are all able to be updated to the current version, iPadOS 13.5.1.

It depends on your iPad model, and its compatibility with most recent iOS/iPadOS versions.

Settings>General>About>Model Number... copy&paste it here, and you'll get the answer.

Regards

Giulio