Old kernel, unused packages, package configs are not necessary useful for your system. Most of the user ignore about it. There are lots of way to do it. If you are smart enough, you can use few lines of commands to do it. I am not sharing any handy commands to perform these task in this post. You can do it with an application called Ubuntu Tweaks in simple easy way with fancy GUI.
sudo add-apt-repository ppa:tualatrix/ppa sudo apt-get update sudo apt-get install ubuntu-tweak
2. Run Ubuntu tweak. Go to Janitor, on the left side you will see system, where you can easily find a check box option to remove you old kernel, package configs, unneeded packages.
3. Choose the stuff that you want to clean. Ubuntu tweaks will list down everything from the category you have choosen
4. click on clean at right hand side which will complete rest of the process