I'm not running Win10 right now (still on 7) so I can't guarantee these still work, but they did in earlier builds of Win10 so you'll have to try to see if it still works in the RTM build.
Registry: HKEY_CURRENT_USER \ Control Panel \ Personalization \ Desktop Slideshow there should be a value there that's set in milliseconds that can be changed to whatever you desire.
Assuming that's been removed you could also try modifying the theme directly. C:\Users\USERNAME\AppData\Local\Microsoft\Windows\Themes. Open the theme in a text file and search for interval. Same thing, listed in milliseconds change as desired.
1 Hour = 3600000 milliseconds.
Not sure why they would remove this from the GUI, but I would think it likely that either of the above options still works