having trouble to install cuda8.0 after downgrading from cuda9.0

I keep getting the error below when I try to install cuda8.0 back.

I am wondering how I can rem the Hash sum mismatch error.

Get:28 http://113.215.21.30:80/1Q2W3E4R5T6Y7U8I9O0P1Z2X3C4V5B/developer.download.nvidia.com/compute/cuda/repos/ubuntu1604/x86_64  Packages [78.7 kB]
Err:28 http://113.215.21.30:80/1Q2W3E4R5T6Y7U8I9O0P1Z2X3C4V5B/developer.download.nvidia.com/compute/cuda/repos/ubuntu1604/x86_64  Packages
  Hash Sum mismatch
Fetched 66.8 kB in 15s (4,190 B/s)
Reading package lists... Done
E: Failed to fetch http://113.215.21.30:80/1Q2W3E4R5T6Y7U8I9O0P1Z2X3C4V5B/developer.download.nvidia.com/compute/cuda/repos/ubuntu1604/x86_64/Packages.gz  Hash Sum mismatch
E: Some index files failed to download. They have been ignored, or old ones used instead.

are you working through a proxy server?

It looks like you simply have an unreliable connection to the internet.