If you have logged in as root then you should be able to proceed as described. If not, you will need to switch to the root user with the following command:
This will then require you to enter the password. Once you have done this, you will be logged in as the root user.
Now that you have logged in as root, you can proceed with the following command to update your system:
apt-get update; apt-get upgrade
If there are any updates available, it will ask you if you would like to continue.