In the past it has been possible to perform upgrades to Fedora systems, to move from one release to the next. This was not the case between Fedora 18 and 22 — at least doing an upgrade is not recommended.
Beginning with Fedora 18, the Anaconda installer no longer has an option to perform an upgrade on an existing Fedora Linux system. In addition, the document, Fedora 18 Installation Guide, in Chapter 19 (Chapter 18 in the Fedora 19 Installation Guide) recommends against attempting an upgrade using any means and suggests that complete reinstallations be performed instead, although it does provide information on how to perform an upgrade.
Of course I have always found upgrades to be somewhat more problematic than a complete reinstallation anyway. Upgrades always leave behind little bits of cruft that eventually cause problems.
There is a new upgrade procedure called fedup beginning with Fedora 18. The fedup program does attempt to perform an upgrade of an existing Fedora system but with all of the warnings against performing upgrades it seems a better option to just do the reinstallation. Just be certain you follow the documented recommendations in the appropriate Installation Guide for backups and verification of configuration files regardless of which path you choose.
Starting with Fedora 22 a new upgrade option is available, dnf system-upgrade. I have successfully upgraded from Fedora 22 to Fedora 23 and 24, and then to Fedora 25. This new upgrade function seems to work very well and the resulting upgraded systems are very stable with no noticeable accumulated cruft.