If there is some unrecoverable error that requires a fresh install, you don't want to lose any data.
If you are upgrading from an End of life release, please also see the end-of-life section.
Backup any personal data to an external hard drive or to another machine.
If you need to upgrade across several releases, it is generally recommended to go one release at a time: for example, rather than going directly from Fedora 24 to Fedora 26, first go to Fedora 25 and then from there to Fedora 26.
This tends to reduce the number of package dependency issues you may encounter.
If you are at all unsure in any way, ask for help on a mailing list, forum or IRC before removing packages. That is probably because you have used non-standard repositories or installed non-standard packages manually.
Try to guess which packages cause the problem (or at least is a part of the dependency chain) - uninstall them and try again.It is a good idea to do the upgrade outside the graphical environment.Log out of your graphical desktop and then A small script named fedora-upgrade is available which aims to automate the process outlined below.You may find that a package you care about depends on a package that must be removed for the upgrade to proceed.Usually you will be able to reinstall the important package once the upgrade is complete.You can also find package signing keys for currently-supported releases here. Click Primary (or Secondary, if you are using a secondary architecture), and you will see Get it from: Fedora Project, where Fedora Project is a link.