In the case when you want to remove linked device to your account, there are two ways of doing that:

1. Open to the desired location on your Kitcast dashboard and switch to the Devices tab. Check the device you want to remove and click on the Trashes icon next to it: 

2. If you don't have access to your Dashboard anymore, forgot login data, your trial ended or there any other reasons for that, you can unlink device directly at the Apple TV:

For that use the trackpad on the Apple TV Remote to hold for 6 seconds the touch bar and release. After that choose Unlink the Device option:

After that you can proceed logging into your new account and link a device to it.