1.Add GIMP PPA
Open terminal from Unity Dash, App launcher, or via Ctrl+Alt+T shortcut key. When it opens, paste below command and hit Enter:
sudo add-apt-repository ppa:otto-kesselgulasch/gimp
2.Install or Upgrade the editor.
sudo apt-get update
sudo apt-get install gimp
3.(Optional) Uninstall.
Just in case you want to uninstall or downgrade GIMP image editor. Use Software Center to remove it, or run below commands one by one to purge PPA as well as downgrade the software:
sudo apt-get install ppa-purge
sudo ppa-purge ppa:otto-kesselgulasch/gimp