How can I do update version in iPad Air 2

See this, Update your iPhone, iPad, or iPod touch - Apple Support

If you have an iPad Air that is still stuck on iOS 12, you own a 1st generation, 2013 iPad Air and NOT an iPad Air 2.

The 1st gen iPad Air cannot upgrade beyond some version of iOS 12.

Sorry.

If you need/want an iPad with an up to date iPadOS with access to current, updated apps, then you really need to seriously consider a new iPad model OR a much “newer” , used iPad model capable of running the latest iPadOS versions. There are plenty of newer, older iPad models that can run the latest, current versions of iPadOS.

Sorry & Best of Luck to You!