Q: How do I transfer data from an iPad Air to an iPad Air 2 celluar

This explains how to do it.

Transfer content from an iPhone, iPad, or iPod touch to a new device - Apple Support

You make a iTunes or iCloud back-up then you set-up the new iPad using that backup. My I suggest that you don't sell the Air until you set-up the Air2. That way tou can check to see if you have all your data.