Upsell items are products that are closely connected to the currently viewed product. You customer may also wish to purchase this other product, thus "upsell".
To navigate to an item's related upsell items, go to More, Upsell.
Adding, deleting, navigating, and reciprocated is the exact same as in Item Substitutions. See that article for more information.
The advantage of having a Substitute and Upsell data is during the sales process. On an Estimate Order or a Sales Order you have the option of selecting a substitute item or an upsell item.