To configure Ubuntu systems to update from mirrors.iu13.net, you need to add mirrors.iu13.net to your apt sources list.
The script provided below (tested on Ubuntu 18 through 24) automatically backs up your current apt sources list and adds entries for mirrors.iu13.net. It does not remove your current sources.
sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak ; echo "$(cat /etc/apt/sources.list.bak | sed -n 's/http.*archive.ubuntu.com/https\:\/\/mirrors.iu13.net/p' | cat - /etc/apt/sources.list.bak)" | awk '!seen[$0]++' | sudo tee /etc/apt/sources.list
If you need to undo the changes, execute this command:
sudo cp /etc/apt/sources.list.bak /etc/apt/sources.list
If you just want to download the latest Ubuntu ISO, see ubuntu-releases instead.