W: The repository 'http://ppa.launchpad.net/gnome-terminator/ppa/ubuntu yakkety Release' does not have a Release file. N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use. N: See apt-secure(8) manpage for repository creation and user configuration details. E: Failed to fetch http://ppa.launchpad.net/gnome-terminator/ppa/ubuntu/dists/yakkety/main/binary-i386/Packages 404 Not Found E: Some index files failed to download. They have been ignored, or old ones used instead.
Simply remove the repository that give you the problem. You can use ppa purge from the terminal or use the gui.
Install ppa-purge with the command:
sudo apt install ppa-purge
After that you have installed ppa-purge give this command:
sudo ppa-purge http://ppa.launchpad.net/gnome-terminator/ppa/ubuntu/dists/yakkety/main/binary-i386/Packages
After that ppa-purge have finished and removed the repository that is giving you problem, you can update without problems.