How to update from 12.5 to 14 or 15 operating system an iPad air

As Niel said, if you're running 12.5 it means its first gen, not second gen. You can tell because the 2nd gen has Touch ID, while the first gen doesn't.

An unit running any version of 12.5 isn't capable of 13 or above and won't be a second generation iPad Air. To update an unit to the latest compatible release, follow the instructions in the Related Article above.

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Are you sure? Could you provide a picture of your home button?

iOS 12.5 was not released for the iPad Air 2 (the one with Touch ID). Either you have 12.4.x or you don't have Touch ID.