iOS 10
I don’t see iOS 10 on my iPad Air 2
The current version of iOS for an iPad Air 2 is 11.2.6. It isn't possible to install an earlier version.
In case you aren't sure of the model you have, if your iPad is running iOS 9.3.5 and won't update further, that would indicate that it is an iPad 2 rather than iPad Air 2. The following may also help if you are unsure about the model: Identify your iPad model - Apple Support