How to update Docker Ubuntu 16?

Question

ubuntu

I want to update docker.
I do everything according to the official doc: https://docs.docker.com/engine/install/ubuntu/

After

sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux ubuntu $(lsb_release -cs) stable"

do
sudo apt-get update

and get:

W: The repository 'https://download.docker.com/linux ubuntu 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 https://download.docker.com/linux/dists/ubuntu/xenial/binary-amd64/Packages  404  Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.

what wtf? how is it treated?