Edit /etc/apt/sources.list on your Ubuntu machine and replace the default repositories:
deb http://ubuntu-mirror.uknode.co.uk/ubuntu jammy main restricted universe multiverse
deb http://ubuntu-mirror.uknode.co.uk/ubuntu jammy-security main restricted universe multiverse
deb http://ubuntu-mirror.uknode.co.uk/ubuntu jammy-updates main restricted universe multiverse
deb http://ubuntu-mirror.uknode.co.uk/ubuntu jammy-backports main restricted universe multiverse
For Ubuntu 24.04 (Noble), use:
deb http://ubuntu-mirror.uknode.co.uk/ubuntu noble main restricted universe multiverse
deb http://ubuntu-mirror.uknode.co.uk/ubuntu noble-security main restricted universe multiverse
deb http://ubuntu-mirror.uknode.co.uk/ubuntu noble-updates main restricted universe multiverse
deb http://ubuntu-mirror.uknode.co.uk/ubuntu noble-backports main restricted universe multiverse
Then run:
sudo apt update
sudo apt upgrade