If you encounter an error with entering Fastly CDN keys on Upsun and end up with a 404 error, here's what you do. 

• In your browser, with the error active, open Inspect mode
• Open the Storage tab 
• Inside Storage, open Local Storage
• Locate the Fastly item (it may be called fast.upsun or similar), right click, and delete

You should then have a new, open form for re-entering your keys