Some index files failed to download

Hello all,

Having done a clear installation from Jessie to Stretch, when trying to 
"sudo apt-get update" I got the following error message:

W: The repository 'http://deb.debian.org/debian stretch/updates Release' 
does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore 
potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user 
configuration details.
E: Failed to fetch 
http://deb.debian.org/debian/dists/stretch/updates/main/binary-amd64/Packages 
  404  Not Found [IP: 2a04:4e42:1b::204 80]
E: Some index files failed to download. They have been ignored, or old 
ones used instead.

I've tried to find a workaround in web, unfortunately with no luck.

Any ideas please?

Thanks,
Pavlos