Click on Listings.
Click on the Sales Channel you want to cancel the listing on.
Click on Manage Listing.
It will now bring up all of the listings for the Sales Channel but you can filter the products by selecting the listing template that was used, by category or even just by searching for the product.
When you have found the item that you want to relist click on Relist on the right hand side of the screen.
Congratulations you have Relisted an item on a Sales Channel.