There is only one way that I would consider safe, and reliable. That would be to obtain a new installation CD or DVD, and do a clean install.
Upgrading from one release to another is problematic, at best. Jumping many levels is not possible.
Depending on the way you set up partitions, this is not as bad as it may sound. If your /home/ partition is not part of the root partition, then you do not need to format /home/. Home is where all your user data should be stored; unless you made some other directory on another disk.
If home is part of the root partition, you will have to back up all your data first. You should have a backup any way.
You can download the latest version of Ubuntu, or any other distro you like. Burn a CD(s) or DVD, and install. Take a close look at some of the threads for Ubuntu, they have made some significant changes to the desktop lately, and not all people like it. I'm running 10.04 on my netbook, and have no plans to replace it with any of the 11 versions. 10.04 is LTS ( Long term support ). 11 versions are being changed much more frequently. So if upgrading is an issue, give that some thought.
Hope this helps.