Software Upgrade

You don't. If your iPad Air is running 12.5.5 it means it's a First generation iPad Air from 2013, and lacks the hardware to run anything newer.

If your bank app requires a newer iOS then you'll need a newer iPad.

An iPad Air second generation or later, a regular iPad 5th generation or later, an iPad mini 4th generation or later or any iPad Pro can run iOS 13 or later.

The first generation iPad Air, iPad mini2 and mini3 can only be updated to iOS 12.5.5.

Apple ended major update support for these models of iPad in September 2019 - although they since continue to receive “point” security updates - the most recent being released just recently. 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 iPadOS15.

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

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

edge-2021 wrote:

My iPad shows iOS 12.5.5 up to date.

But the bank app NO LONGER SUPPORT this version.

How do I upgrade it ?

You would need to purchase a newer ipad that supports your bank app.