"Craft CMS does not support backtracking to this version. Please upload Craft CMS 0 or later." I have a problem related this and this question. I began updating a long time ago (2017?) and got the error "Craft CMS does not support backtracking to this version. Please upload Craft CMS 0 or later" when I try to access the site or admin. I'm just now trying to fix it. I have tried replacing the app folder with a clean craft 2 version and I'm getting a blank white screen on everything. 

I run a local install and a live install from the same DB.

What else can I try?
Turns out I hadn't fully uploaded the app folder when I tried to access it earlier. Tried again, made sure it was finished before renaming and it worked fine.