How do I cancel an online Apple trade in that’s scheduled for pick up?

Hello ~ If you have not sent it just don’t send it or contact Apple Support. You can do that as a courtesy by going to upper right hand corner and click on Get Support or:

How to Contact Us - Apple

If you send it in there is no way to stop the trade-in.

~Katana-San~

See the following from https://www.apple.com/shop/trade-in

Can I cancel a trade-in?

If you haven’t dropped off or shipped your device, you can cancel your trade-in by simply keeping your device.

If you’ve already sent us your device, the trade-in can’t be cancelled. If you receive a revised trade-in value after the inspection of your device which is lower than your originally quoted value, you can then choose to reject it.

If you mean that it is scheduled for pickup by FedEx (or similar), contact them.