How can I trade in for my old iPad Air?

Hi, I want to ask the process of trade in because it’s the first time to use this service.

I want to buy iPad Air 5, so I want to trade in my old iPad Air 4. If I choose trade in online, do I need to send my old iPad Air first then I can get the new one? I wonder how transfer data to new device during the trade in.

[Re-Titled by Moderator]