I recommend removing the previous version of GIMP to avoid any conflict. So remove GIMP before proceeding.
sudo apt-get remove gimp
Now add GIMP PPA to your system
sudo add-apt-repository ppa:otto-kesselgulasch/gimp
And now run these commands:
sudo apt-get update
sudo apt-get install gimp
You should be running GIMP 2.8 on your Ubuntu system. If you are using a lot of plugins or scripts be informed that a lot of plugins and scripts have not been upgraded to work with GIMP 2.8. Your plugins/scrips may not work so if you are dependent of these scripts you may avoide upgrading to the version 2.8. We will be publishing a detailed review of GIMP soon.
Call for writers/bloggers. If you are a GIMP user and want to share your tricks, tips and tutorials with users you can send your articles to us at email@example.com.