How can I install gutsy using apt-get?

I would suggest you don't if you can spare a little space on your drive for another partition.

The upgrade solution that works well for me is having two system partitions.

 I'll install Ubuntu on one of them (with /home located in it) and then when a new version is released (usually I can't wait and get the earlier pre-releases) I install it to the other partition (provided it works well enough from the live CD).

 I will then recreate the users to login to the machine and make backup copies of the default home folders on the new system (frank copied to frank-org).

Then I'll copy the user folders from the old (previous) home folder to the new, overwriting those new folders.

Most apps will take the old configurations and update them, and to the users the upgrade is nearly transparent as all of their configurations are similar.

The main benefit of this is that if something horrible happens that you do not forsee, reverting back is as simple as selecting the previous install from grub boot menu with everything exactly as it was and unchanged!

That said, when Gutsy final is released officially you will see an upgrade prompt when you use you update manager which will upgrade you to Gutsy via apt.

There are ways that you can do it beforehand following this guide:
http://ubuntuguide.org/wiki/Ubuntu:Feisty#Updates_and_Upgrades

I'm afraid I don't have enough space to use for a partition, but thanks. Another problem I have is that I don't know which are the user files and folders(so that I can backup them) and I end up losing all my configurations and work that I have done to the old system. For example the reason I want to try upgrading now instead of waiting the final release, is because I have trouble with my Xorg (I donno if I even say that right) and I don't know how to reinstall it. I tried using the backup xorg.conf file but it didn't work.

The process is pretty straight forward.

1) in your favorite editor, with admin rights, edit /etc/apt/sources.list (e.b '' sudo vi /etc/apt/sources.list")
2) Replace all occurrences of the word "feisty" with "gutsy" (e.g. in vi , ":g/feisty//s/gutsy/g")
3) "sudo apt-get update"
4) "sudo apt-get dist-upgrade"

Just be careful. While Gutsy is coming at us quickly, this is still s development release, and so things still can, and will, break between now and go-live. Just remember, if it breaks, you get to keep BOTH pieces, and should be helping out by filing bug reports :)

            Regards,
                   --Matt

your user configurations are neatly stored in /home/<user name> There is a folder there for each user. In this are the prefs and files for your desktop, and configurations for apps that you run from the desktop (openoffice, firefox, gaim/pidgin vlc, and the like ) You may also find that a lot of the work that you did to configure the system side ( /etc/ ) has been either implemented or fixed in the latest version of Ubuntu. For example some of the xorg.conf NVidia driver options I had to put in manually are now default.

First replace just the first feisty with gutsy (the main repository). Update and upgrade. Otherwise old apt-get will probably stop with a segmentation fault when trying to process new universe package list (did for me).

After upgrade you will probably want to run System-->Administration-->Screens and Graphics, to configure your display.

I wouldn't recommend upgrading to a beta unless you're really comfortable with linux. You're likely to get into some trouble.