How do I download and install a newer version of iOS on my ipad?

It depends on hardware compatibiliy of your model.

If what you wrote on your product info is correct, your iPad Air cannot support iOS versions past 12.4.9.

And these are the instructions to update your device:--> Update your iPhone, iPad, or iPod touch – Apple Support

Regards

Giulio

If you actually have an iPad Air, if its an older, 2013 iPad Air 1st generation, you can only upgrade that iPad to the latest version of iOS 12, which currently is iOS 12.4.9.