The item library is the heart of a retail site. Everything else, layouts, inventory, reporting, hangs off it.
In the Merchant Portal, click Items in the navigation menu on the left, then click Item Library.
Go to Items > Item Library, open the item and change what you need.
Go to Items > Item Library, select the item and delete it. Prefer marking an item inactive over deleting it if it has sales history.
Marking an item inactive takes it out of selling without destroying its history. This is the right move for seasonal stock and discontinued lines.
From Items > Item Library you can move an item from one category, subcategory or group to another.
For a new site, load the library with the catalog upload rather than by hand. The Genius Help Center publishes a Catalog Upload Guide with the template and its rules.
Source: Global Payments Genius Help Center for Genius for Retail. Confirm menu paths against your own portal, as some options vary by package and region.