How to remove cuda-9.0 and install cuda-8.0 instead?

I got the exact same issue and managed to fix it. First observation was that the cuda-8 related entries in sources.list.d/ were commented out, but removing these and doing the dpkg --install of the cuda-8 repo did not repopulate the sources.list.d/ entries. What worked was to purge the 'dpkg' install (/var/cuda*) manually and doing the 'dpkg --install' again.

step by step:

• dpkg -l | grep cuda- | awk '{print $2}' | xargs -n1 sudo dpkg --purge
• dpkg --install cuda-repo-ubuntu*-8.0-local*.deb
• sudo apt-get update
• sudo apt-get install cuda

There's actually a script in /usr/local/cuda-9.x/bin/ which performs the uninstall process for you.

https://devtalk.nvidia.com/default/topic/1024342/cuda-setup-and-installation/unable-to-uninstall-cuda-9-0-completely-and-install-8-0-instead/

It works for me. It might be helpful for you too.