Follow below steps to upgrade to F28.
1. Update software and back up your system
Before you do anything, you will want to make sure you have the latest software for Fedora 27 before beginning the upgrade process. To update your software, use GNOME Software or enter the following command in a terminal.
sudo dnf upgrade --refresh
Additionally, make sure you back up your system before proceeding. For help with taking a backup, see the backup series on the Fedora Magazine. Or use Veeam Free Backup for Linux, I am personally using this one.
2. Install the DNF plugin
Next, open a terminal and type the following command to install the plugin:
sudo dnf install dnf-plugin-system-upgrade
3. Start the update with DNF
Now that your system is up-to-date, backed up, and you have the DNF plugin installed, you can begin the upgrade by using the following command in a terminal:
sudo dnf system-upgrade download --releasever=28
This command will begin downloading all of the upgrades for your machine locally to prepare for the upgrade. If you have issues when upgrading because of packages without updates, broken dependencies, or retired packages, add the ‐‐allowerasing flag when typing the above command. This will allow DNF to remove packages that may be blocking your system upgrade.
4. Reboot and upgrade
Once the previous command finishes downloading all of the upgrades, your system will be ready for rebooting. To boot your system into the upgrade process, type the following command in a terminal:
sudo dnf system-upgrade reboot
Your system will restart after this. Many releases ago, the fedup tool would create a new option on the kernel selection / boot screen. With the dnf-plugin-system-upgrade package, your system reboots into the current kernel installed for Fedora 27; this is normal. Shortly after the kernel selection screen, your system begins the upgrade process.
Now might be a good time for a coffee break! Once it finishes, your system will restart and you’ll be able to log in to your newly upgraded Fedora 28 system.