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.

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