Alternatively, use the automatic Software Updater. Go to Menu --> System Tools --> Software Updater. Check for updates and if the button is available (meaning there are updates to install), then install.
For the moment, if you installed the Microsoft "core" fonts (Andale Mono, Arial, Arial Black, Comic Sans MS, Georgia, Impact, Tahoma, Times New Roman, Trebuchet, and Verdana), you may get an error after updating because the Ubuntu repository has version 3.4 of MS Core Fonts, but it's up to 3.6.
To update and/or avoid or "fix" the timed popup (saying, "This version of XScreenSaver is very old! Please upgrade!"), update XScreenSaver from Debian's repository.
In Firefox or any web browser, type (or copy and paste) in the address bar: ftp.us.debian.org/debian/pool/main/x/xscreensaver/ and hit [Enter]. You'll see a list of many XScreenSaver packages. Update yours by choosing the replacements with the highest number. Click the correct packages for your system (amd64 or i386):
Go to Downloads (or wherever you downloaded these packages) and double-click them one at a time to install with GDebi. Ignore the pop-up warning that an older version is available in the software channel by clicking the "Close" button. Then click "Install Package." Repeat for each package you downloaded.