Open terminal and check your current kernel:
uname -r
DO NOT REMOVE THIS KERNEL!
Then, type the command below to list all installed kernels on your system.
dpkg --list | grep 3.16.
Find all the kernels that lower than your current kernel. When you know which kernel to remove, continue below to remove it. Run the commands below to remove the kernel you selected.
sudo apt-get purge 3.16.X-X-generic
Open a terminal.
Remove outdated packages from cache:
sudo apt-get autoclean
Remove every package from the cache:
sudo apt-get clean
Remove installed packages as dependencies which have been unused
sudo apt-get autoremove
«apt-get autoclean» does not just delete outdated packages, it removes them from the cache, there is a difference.
May I suggest changing the description from «delete outdated packages» to «delete outdated packages from cache».
«clean» is a harder version of «autoclean»: it removes every package from the cache, not just those that can no longer be downloaded/installed (what you call «outdated» in this text).