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  Packages [78.7 kB]
Err:28  Packages
  Hash Sum mismatch
Fetched 66.8 kB in 15s (4,190 B/s)
Reading package lists... Done
E: Failed to fetch  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.