Make sure your current system is up-to-date and backed up

First up, ensure that you have the most up-to-date version of Fedora 21 by updating your system using the Software application, or running the following command in the terminal:

sudo dnf update

Install the FedUp tool

Now, open up a Terminal and install FedUp package by using the following command line:

sudo dnf install fedup

Start the update with FedUp

Now your system is fully up-to-date, start the upgrade using the following command:

sudo fedup --network 22

This command will tell FedUp to download all the appropriate packages from the Fedora repositories, and prep your system for the upgrade.

Reboot, and upgrade

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Once the FedUp command from step 3 has completed without errors, reboot your system. In the Fedora Boot Menu (i.e. grub), there will be a menu item “System Upgrade”, select that and press enter.

Further Information

For more detailed instructions on using FedUp for upgrading (including using a downloaded ISO or another installation source other than the network), check out the FedUp wikipage. This page also has details on issues that you may encounter during an upgrade.

References

How to Upgrade from Fedora 21 to Fedora 22