Using the terminal:
Update the Package Index: The APT package index is essentially a database of available packages from the repositories defined in the /etc/apt/sources.list
file. To update the local package index with the latest changes made in repositories, type the following:
sudo apt-get update
Upgrade Packages: Over time, updated versions of packages currently installed on your computer may become available from the package repositories (for example security updates). To upgrade your system, first update your package index as outlined above, and then type:
sudo apt-get upgrade
To get a source:
apt-get source <package name>