There are several ways to update your Ubuntu. Here is the details.
As you are new to Ubuntu. May be you are not so comfortable with command line. In that case, this way will be best for you. The easiest way is using Update Manager. Normally if update manager gets any update, it'll notify you automatically. Otherwise, you can click on the GEAR icon on the top right of your screen, it will show you a menu. If your system has update then number 4 option will show "Update Available", otherwise it'll show "Software up to date". Click on this option, it'll open the Update Manager of Ubuntu. It'll show you the details of the updates available or not. Install the updates from here as your wish.
Press ctrl+alt+T. It'll open the terminal. Then type
sudo apt-get update && sudo apt-get upgrade
It'll ask for your password. Give the password and press enter. Packages will be updated automatically.