I have tested this method on several machines, but if you have problems, please let me know. Always remember backup, before upgrade!
1. Before Upgrade
1.1 Backup Your Data
Before doing anything, do full system backup or backup at least your important data: documents, pictures, videos, config files, log files, databases etc. This is very important step, because if something goes wrong and you have to do fresh install, then you can easily restore your data.
2. Upgrade Fedora 25 to Fedora 26 using DNF
2.1 Change root user
su - ## OR ## sudo -i
2.2 Update Fedora 25 Packages
2.3 Reboot Your System
Just make sure that you are running latest kernel.
2.4 Install/Update dnf-plugin-system-upgrade package
It’s probably installed already.
dnf install dnf-plugin-system-upgrade --best
2.5 Start Upgrading Using DNF
dnf system-upgrade download --releasever=26
When this finish, check if there is any errors. You might need
options to get all packages downloaded.
2.6 Start (F25 to F26) Upgrade Process
dnf system-upgrade reboot
2.7 Grub 2 Menu Boot with Latest Kernel
2.8 Starting System Upgrade
2.9 dnf-plugin-system-upgrade Upgrading Fedora 25 to Fedora 26
3. Fedora 26 Screenshots
3.1 Fedora 26 GDM Login Screen
3.2 Fedora 26 Plain Desktop Gnome Shell 3.24.2
3.3 Fedora 26 and Gnome 3.24.2
4. Fedora 26 Clean Up Post Upgrade (as root user)
4.1 Rebuilding the RPM DB
4.2 Do Distro Sync
dnf distro-sync --setopt=deltarpm=0
4.3 Check All Your Config Files
Use rpmconf to find .rpmnew, .rpmsave and .rpmorig files. You can keep current version, place back old version, watch the diff or merge.
dnf install rpmconf rpmconf -a