When a new version of Ubuntu rolls out, you might want to upgrade your older Ubuntu to that version. There are many ways to do this.
If your current Ubuntu is not too old, then Ubuntu might support an upgrade to the latest version. There are two types of upgrade possible: to any latest version or to only the latest long-term support (LTS) version.
To do this, open Software Updater or run
update-manager at the shell. In the dialog, enable Updates → Notify me of a new Ubuntu version.
It will check the Ubuntu repositories and if your Ubuntu can be upgraded you will be notified in a while.
Note that this upgrade will take a long time, it took 6 hours on my system. It will try to upgrade all your applications and libraries and finally the OS. So, if you want to speed up the process, just remove all applications and libraries that you can beforehand. You can reinstall them after the upgrade.
An upgrade actually takes a very long time to finish. Since Ubuntu installs in as little as 15 minutes on current computers, I do not think an upgrade is always worth the time. Instead make sure that your
/home are on different partitions. In such a setup, you can just do a fresh install of the new Ubuntu version by wiping out the
/ partition and by requesting it to use the existing
/home partition. This is one of the useful features of Linux installations and I highly recommend keeping these partitions separate for this reason on all computers.