To edit a Gem, you first need to open the Gem Page for the piece of content in question. Once there, you’ll see an Edit icon near the top of the page - if you are an Admin, or if you are the user who first added this content to Shelf.
If you do not see this icon, it simply means that you do not have permission to make changes to the Gem.
Once you’re on the gem page, you’ll have the ability to change the following things:
The location where the Gem is saved in Shelf
Rate a Gem
If the content in question has been uploaded directly to Shelf, you’ll also have the option to change the file attachment.