After almost 5 hours of upgrading, my server has been successfully upgraded from Fedora 21 to Fedora 24, which uses the latest stable kernel 4.6. There is a online post demonstrating how to upgrade from Fedora 21 to 23 using fedup. This post talks about Fedora upgrading from 21 to 24 using dnf. NOTE: please do backup your data before action!
0. yum update
This is usually not a problem for Fedora 21, whose support has expired for a long time. Anyway, run it just in case.
1. dnf
According to the Fedora official wiki (https://fedoraproject.org/wiki/DNF_system_upgrade), dnf is recommed for system upgrade. Apparently, fedup has been ditched. Here what we need are 3 dnf commands:
sudo dnf upgrade --refresh
sudo dnf install dnf-plugin-system-upgrade
sudo dnf system-upgrade download --refresh --releasever=24
The last dnf command should list any error, which blocks the upgrade. The errors I have encountered were obsolete packages which are not supported in Fedora 24 repo. As you can tell, the only way to move the upgrade is to remove all these obsolete packages, using “yum remove” + unsupported package name reported by dnf.
Once all the errors are cleaned, dnf is able to download all the required packages for Fedora 24. On my server, it was about 4GB. So, you need at least some GB left to hold all these new packages. More important, dnf requires another 5GB under root during the package installation. Make sure you make dnf happy.
2. Keys
Before dnf was able to install all new downloaded packages, I got such an error:
Couldn’t open file /etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-24-x86_64
There is a bug report talking about the possibilities of this issue and corresponding fixes (https://bugzilla.redhat.com/show_bug.cgi?id=1044086#c11). However, if you find manual key importing does not work, go and take a look at /etc/pki/rpm-gpg directory. What happened to my server was simply no any key file for Fedora 24. Oops. The fix is also easy – creating the key files by ourselves. Go to https://getfedora.org/keys/ and find the key files (primary/secondary). Create these key files and symlink the x86_64 (arch of my server) with the primary. That’s it.
3. dnf again
Reboot the machine to start the upgrade:
sudo dnf system-upgrade reboot
Hint: yum is now deprecated. Run “dnf update” once you are into the new system.
davejingtian.org 
Hi Dave,
” The fix is also easy – creating the key files by ourselves. Go to https://getfedora.org/keys/ and find the key files (primary/secondary). Create these key files and symlink the x86_64 (arch of my server) with the primary. That’s it.”
Can you tell me more detail? How to do like you wrote?
Thanks,
Thuan
Go and take a look at /etc/pki/rpm-gpg directory. You will find the existing keys. Download Fedora 24 keys from the web site I posted above, e.g, here is the primary key file for Fedora 24 – https://getfedora.org/static/81B46521.txt. Save this key file in he rpm-gpg directly according to the same naming style. Create symlink named RPM-GPG-KEY-fedora-24-x86_64 pointing to the primary (if I recall).
Hint:
No need to remove unsupported packages manually. Simply add the –allowerasing option when downloading and it will automatically remove packages with broken dependencies.
Hello all,
I am new in fedora and this what I did but is still did not work.
create file 81B46521.txt and copy information from https://getfedora.org/static/81B46521.txt.
then make simbolic link as you suggested.
what was done
[root@home rpm-gpg]# cat 81B46521.txt
pub 4096R/81B46521 2015-07-25 Fedora (24)
—–BEGIN PGP PUBLIC KEY BLOCK—–
Version: GnuPG v1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==
=LPDE
—–END PGP PUBLIC KEY BLOCK—–
Create sibolic link
[root@home rpm-gpg]#
===
-rw-r–r– 1 root root 1740 Nov 21 14:43 81B46521.txt
lrwxrwxrwx 1 root root 44 Nov 21 14:44 RPM-GPG-KEY-fedora-24-x86_64 -> RPM-GPG-KEY-rpmfusion-free-fedora-24-primary
[root@home rpm-gpg]#
Error message
SKIPPED] youtube-dl-2016.10.25-1.fc24.noarch.rpm: Already downloaded
warning: /var/lib/dnf/system-upgrade/adobe-source-han-sans-tw-fonts-1.004-3.fc24.noarch.rpm: Header V3 RSA/SHA256 Signature, key ID 81b46521: NOKEY
GPG key at file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-24-x86_64 (0xB7546F06) is already installed
Error: The GPG keys listed for the “Fedora 24 – x86_64” repository are already installed but they are not correct for this package.
Check that the correct key URLs are configured for this repository.
Failing package is: adobe-source-han-sans-tw-fonts-1.004-3.fc24.noarch
GPG Keys are configured as: file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-24-x86_64
Could you please help ?