Remove the Ethereum repository first:
sudo add-apt-repository --remove ppa:ethereum/ethereum
sudo apt update
Remove imported GPG key:
sudo apt-key del 2A518C819BE37D2C2031944D1C52189C923F6CA9
Import GPG key of the Ethereum repo again:
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 2A518C819BE37D2C2031944D1C52189C923F6CA9
Add repository again:
sudo add-apt-repository -y ppa:ethereum/ethereum
sudo apt update
UPDATE
Remove the Ethereum repository again:
sudo add-apt-repository --remove ppa:ethereum/ethereum
sudo apt update
Remove imported GPG key:
sudo apt-key del 2A518C819BE37D2C2031944D1C52189C923F6CA9
Try
gpg --keyserver keyserver.ubuntu.com --recv-keys 2a518c819be37d2c2031944d1c52189c923f6ca9
and next
gpg --export --armor 2a518c819be37d2c2031944d1c52189c923f6ca9 | sudo apt-key add -
Add repository for Ubuntu 16.04:
echo -e "deb http://ppa.launchpad.net/ethereum/ethereum/ubuntu xenial main" | sudo tee /etc/apt/sources.list.d/ethereum-ubuntu-ethereum-xenial.list
Update apt:
sudo apt update
Install software you want from the Ethereum repository.