Anaconda, the Fedora installer does have not any in-built upgrade functionality in Fedora 18 or above releases. Currently, Fed Up is capable of upgrading Fedora 17 installs to Fedora 18 using a networked repository, similar to how Pre Upgrade worked.
More methods for upgrade are currently planned and this page will be updated as those features are completed.
When I made that comment I meant one thing and the phrase "rolling releases" really means something else.
When I wrote that I meant "rolling release" to mean that you could now roll from one point release of Fedora (say 17) to version 18.
Fed Up (FEDora UPgrader) is the name of a new system for upgrading Fedora installs in Fedora 18 and above releases.
It replaces all of the currently recommended upgrade methods (Pre Upgrade and DVD) that have been used in previous Fedora releases.
See @Joel Davis' answer for this technique as well.
NOTE: This is no longer an option starting with Fedora 18 though so you've been warned. New in Fedora 17/18 is a tool called Fed Up (FEDora UPgrader) which purports to do "dist-upgrades" similar to Debian/Ubuntu distros.
However in looking up the description of what "rolling releases" actually means on wikipedia, I'm now more educated on the subject. a rolling release or rolling update development model refers to a continually developing software system; this is instead of a standard release development model which uses software versions that must be reinstalled over the previous version.
Even in rare cases where the development branch is a rolling (versus the more common cyclical) development branch, this does not make the distribution rolling.
Unlike standard release distributions, rolling release distributions do not have development branches.
Figure CThe first thing that hit me was how much smoother and faster Fedora ran.
In fact, the difference between 25 and 26 was significant (Considering how polished the previous release was, that's saying something.) In fact, this might have been the smoothest major upgrade of any operating system I have ever experienced, bar none. At least from my perspective, I would say upgrading Fedora 25 to 26 might be the first time I've ever said the process could end with as stable a desktop as would a clean install. That would depend upon what purpose the installation served. In the end, Fedora 26 is a slick, stable, and gorgeous desktop operating system.
This will take can take some time (but not nearly as long as the actual upgrade); so either sit back and enjoy the progress or go take care of some other task.