I Deleted Something in the File Manager, but I Don't See the Extra Disk Space

When you delete files in the file manager, they are copied to the wastebasket. To finally delete them, right-click the wastebasket in the bottom right-hand corner of the screen, and click Empty the Wastebasket. A dialog box pops up asking you to confirm the deletion. Click the Empty the Wastebasket button, and the files are removed.
Another Version of Ubuntu is Out-How Do I Upgrade to It?

If a new version of Ubuntu is released, the update manager will notify you via an upgrade notification pop-up bubble. To upgrade, click the Upgrade button, and follow the instructions.

If you want to manually upgrade, first find out the codename for the next version of Ubuntu (in the same vain as Warty, Hoary, Breezy, and Dapper). Now open /etc/apt/sources.list, and replace the word "dapper" with the new codeword. Now run:

foo@bar:~$ sudo apt-get updates

This command updated your package list. Now upgrade the system:

foo@bar:~$ sudo apt-get dist-upgrade

You are asked if you wish to continue. Press Y, and then press Enter to continue. Your entire system is now upgraded.

As Usual...

Remember to back up all your important files before doing a distribution upgrade!