To delete a channel you've created, first make sure you're switched into the admin mode for your organisation/ecosystem.
Next, make sure you're looking at your dashboard:
From the options here, click on 'Manage Experiences':
You'll default here to seeing experiences you've created, so switch the view to show channels:
Lastly, find the channel you wish to delete, click the '...' menu, and select 'Delete'. Once you've done that, you may just need to refresh the page to see your changes go through.