Ubuntu software packaging system can accumulate unused packages or temporary files. These temporary files, also called caches, contain package files from all of the packages that you have ever installed. Eventually, this cache can grow quite large. Removing them allows you to reclaim space on your computer’s hard drive for storing your documents, music, photographs, or other files.
To clear the cache you can use either the clean or the auto-clean op for a command-line program called apt-get. The clean command will remove every single cached item, while the auto-clean command only removes cached items that can no longer be downloaded (these items are often unnecessary).
To run clean go to Applications –> Accessories –> Terminal and Type:
$ sudo apt-get clean
and it will ask password and give the password
packages can also become unused over time. If a package was installed to assist with running another program- and that program was subsequently removed-you no longer need the supporting package. You can remove it with autoremove
To run clean go to Applications –> Accessories –> Terminal and Type
$ sudo apt-get autoremove
it will remove the unnecessary packages.