If someone have Microsoft windows laptop to attached iPad Air , how to install latest update on your iPad please. Tell me.

A first generation iPad Air, iPad mini2 and mini3 can only be updated to iOS 12.5.1.

Not sure which iPad you have? Identify your iPad model.

Apple ended major update support for these models of iPad in September 2019 - although they since continue to received “point” security updates. These devices cannot be updated to iPadOS 13 (or any later major versions of iPadOS) as the internal hardware does not meet the minimum technical requirements (CPU and RAM) for newer versions of iOS/iPadOS.

Some App developers have dropped support for iOS versions preceding iOS 13.x for technical reasons (often because APIs or software libraries upon which they rely have been updated in the new version of iPadOS, making their App incompatible with earlier versions). 

Whilst many App titles remain compatible with your iPad, if you want or need to access to Apps requiring the most recent versions of iOS, you’ll need to consider replacement of your iPad with a newer or current model that supports iPadOS14.

Sorry, I appreciate that this may not be what you were hoping to learn.

If your iPad is a newer model - and capable of update - these support pages may be helpful:

Update your iPhone, iPad or iPod touch – Apple Support

If you can't update or restore your iPhone, iPad or iPod touch – Apple Support

If your iPhone, iPad or iPod touch won’t update - Apple Support

If you see an error when you update or restore your iPhone, iPad, or iPod - Apple Support

iOS update and restore errors – Apple Support

I hope this information provides helpful clarity as to your available options - and the potential limitations of your existing iPad.