To edit a slideshow, tap the Slideshow icon in the bottom navigation bar. You'll see a list of your created slideshows.
You'll see options to edit, cast, or delete under each slideshow thumbnail. Tap on the edit (pencil) icon:
Just like creating a new slideshow, from the edit page, you can tap to select or deselect your created signboards to add or remove them from the slideshow. Tap the check mark to save your edits.