sudo apt-get upgrade results in 404 errors for everything
The problem you're facing is basically a problem to reach the IP 91.189.91.26, try restarting your Internet connection and check if you can ping the IP address using this command:
ping 91.189.91.26
In my case it works correctly.
If nothing works for you, try changing the mirror your using to get your updates:
Open your sources.list file using your favorite text editor, in my case nano.
sudo nano /etc/apt/sources.list
Locate the text
http://us.archive.ubuntu.com/ubuntu
and replace it with
http://archive.ubuntu.com/ubuntu
And then run:
sudo apt update && sudo apt upgrade
If it still doesn't work, you might try a distribution update:
sudo apt-get dist-upgrade
If still not working, this is what I've found here
Run the following command to update archive.ubuntu.com and security.ubuntu.com package repository URLs with old-releases.ubuntu.com. Since the normal Ubuntu releases link to the archive.XXX and security.XX URLs, the support will be removed after their live cycle of 9 months and respective repositories moved to old-releases.….
sudo sed -i -e 's/archive.ubuntu.com\|security.ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list
Good luck.
.