Install in terminal

Step 1 : Add repository from official Atom Site
wget -qO - https://packagecloud.io/AtomEditor/atom/gpgkey | sudo apt-key add -
sudo sh -c 'echo "deb [arch=amd64] https://packagecloud.io/AtomEditor/atom/any/ any main" > /etc/apt/sources.list.d/atom.list'
Press Enter if it ask your permission.

Step 2 : Update the Ubuntu System
sudo apt-get update
It will take some time. Be patient.

Step 3 : Install Atom Editor
sudo apt-get install atom

# Install Atom
sudo dpkg -i atom-amd64.deb
# Install Atom's dependencies if they are missing
sudo apt-get -f install

# Install Atom
sudo apt-get install atom
# Install Atom Beta
sudo apt-get install atom-beta