Once an item has been added to a Catalog, you can rename and/or replace the item from the Catalog area - not the Media area.

Open Catalogs from Main Dashboard, Find Catalog containing Media item, Double-Click to open Catalog, Click on the item, Make changes, Click Save.

These are Media items that have been added to Media Catalogs.  

Click on Media Icon, Open appropriate Catalog, Click on the edit icon next to the Media item:

Open

Make changes, Click on the checkmark to Save or X to exit.

Open Media Main Dashboard, Click on the item to delete, Choose Delete.

 *If you delete Media from the Media Main Dashboard, it will be permanently deleted.

Click on the Media icon, Click on Catalog that has the item, Click on the trash can next to the item.

*If you delete a Media item from the Media Shortcut icon it simply removes the item from that Catalog.  However, the item will still exist in the Media Dashboard to use again.  To permanently delete Media items, delete them from the Media Main Dashboard.