In cases where you can upgrade, as opposed to a fresh install, always, always, always back up your important data. That includes:
Upgrading Ubuntu is fairly straightforward and painless. Go to the Ubuntu upgrade page a guide to the process.
In general, you can no longer upgrade Red Hat or CentOS Linux. I'm not sure about Fedora. It may still have a system called "fedup". Now you surely don't want to do a fresh install over your existing installation. I keep two bootable partitions of at least 500 MB on my machines. The remainder of the drive is devoted to LVM (Logical Volume Manager). The volume group is divided into these logical volumes:
When it is time to make a major upgrade, e.g. from v5.x to v6.x, I install to the unused root LV and boot partition, and let it format only those filesystems. That way, if the install process doesn't produce a usable system, I still have an operational partition to use until I have time to fix things. Also, if the new installation is workable, but not quite configured the way you want, you have the old root filesystem to refer to. I used to have a page of instructions on how to do that, but it goes back to 2002 and is badly out of date. If you are really interested in it, hit the Contact link at the top of this page and let me know. Then I will see if I can update it sufficiently to make it useful.
Text vs. GUI upgrade (or install, for that matter)
The default install (or upgrade) uses a graphical screen (GUI) and it generally works fine. If you have a machine with not much RAM, you may need to use the text-mode process instead. Note that this has nothing to do with, and has no effect on what kind of login screen you use (runlevel 3 - text, or runlevel 5 - GUI).
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Bobcat Open Systems, Inc.