How plan: Backup everything that is of importance.
Backup: Of course, you should always have a recent backup, not only before upgrading (Backintime as recommended by @sideburns works really well)
Upgrade usually works just fine using dnf: If you have 3rd party repos other than rpmfusion, I'd recommend disabling them before the upgrade
F24 --> F26: Not a problem: sudo dnf system-upgrade download --refresh --releasever=26.
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