I downloaded a big racing app and later deleted it. About 1 GB of app data is still sitting on the Mi Box though and I don't see any obvious way to delete it.
How do I?
When I click the 2.1 GB of "app data" under storage/settings, I can't go down and see the actual apps and/or delete this data.
The app data is automatically deleted when you uninstall the app.
Install Android Assistant (my bad, Assistant for Android is what they're calling it these days) set it & its automation up, once. It'll help keep your box clean, running fast/smooth & allow you to do a bunch or other things, including what you're looking to achieve here.
One method is to reinstall the game and clear the data through the Mi Box options (Settings, Apps). Then uninstall.
Normally uninstall will get rid of everything, this game must put the save elsewhere.
A file manager may help too, but those are all kinda crappy, so if you add one to find the data, you may want to uninstall that app too.