can I trade in my iMac for a newer version?

Yes, you can trade in your older Mac and the value can be put towards the purchase of a new iMac. See Apple Trade In for the details.

It depends on the model year and condition of the iMac. If it is a very old model, then it likely has no trade-in value however you can still send it to Apple and they will properly recycle it. Also, please follow the steps in What to do before you sell, give away, or trade in your Mac prior to getting rid of it.

If you're thinking of a "swap out" scenario, it's not like that. It's trade in old; purchase new.