In order to upgrade KDE Plasma 4 to KDE Plasma 5 in Fedora 21 KDE you need add repository kde plasma 5 for fedora 21 (dvratil/plasma-5) on your system.
First, install package dnf-plugins-core. you can install it by typing command:
sudo yum install dnf-plugins-core
update your fedora 21 packages to the most recent by entering command:
sudo dnf update
Now, add the repository KDE Plasma 5 for fedora 21 with the following command:
sudo dnf copr enable dvratil/plasma-5
Type the following command, if you want to install/upgrade kde plasma 5 in fedora 21.
sudo yum install plasma-5