There's no way to change which screen comes up first in PowerApps. Please add this feature.
There is a way but it's not really that great. The first screen on the screen list will be the first screen. (Which is somewhat counter-intuitive given that the screen order doesn't really matter since we can only move from screens using the Navigate() function.)
However, I was looking for the OnStart() property, which apparently doesn't re-appear until I've saved and re-opened the app. That should be in the ribbon, by the way.