Ideal procedures

1. Make sure the system is up to date: sudo dnf upgrade --refresh
2. Reboot the system
3. Install the system upgrade plugin: sudo dnf install dnf-plugin-system-upgrade
4. System upgrade to the targeted release: sudo dnf system-upgrade download --refresh --releasever=28
5. There may be some dependency issues. Use --allowerasing or --best wisely.

   

6. Trigger the upgrade: sudo dnf system-upgrade reboot

More Information

https://fedoraproject.org/wiki/DNF_system_upgrade `