My usual procedure:

1. Download ISO of latest version & make cd/dvd. This is your fall back option
in case something doesn't work.

2. Backup your personal data somewhere else; USB-drive, cd/dvd, external HD,
etc...

3. Make a list of everything you want to re-install after the OS is upgraded.

4. Now, you must choose: clean install or upgrade.

4.1. Clean install: Unless you have a separate partition for home, you must
reformat everything. But this is almost guarantied to succeed. Then you put back
your data and re-install your other apps.

4.2. Upgrade: If it works, great! But my experience with it is that you have a
50% chance of failure. When that happens, goto 4.1.