Uninstall via Control Panel > Programs and Features, reboot if asked.
Run the MCPR cleanup tool and reboot.
Reinstall from your online account.
I advise turning off Fast Startup as it can cause problems especially with software that updates frequently but also with Windows Updates, here's a tutorial: Fast Startup - Turn On or Off in Windows 8
I'm curious, most people prefer 10 over 8, why did you go back?