Do you have yumex installed (yum extender)? I think it's installed by default these days, if not, su -c 'yum install yumex'.
Then, activate the repositories for Gnome 3.16 from rawhide. You need to activate two, and accept the authentication keys for them, namely the rawhide:GNOME and the rawhide:GNOME_backports. These can be easily selected from the repositories tab in yumex to install them. After that, do a yum upgrade, and you should be on Gnome 3.16 on your fc21 install.
Don't expect too much yet from Gnome 3.16 though. It still has some strange quirks, especially on Wayland. With Gnome 3.16, gdm is on Wayland, so your login screen pops up when you update gnome packages (which is almost daily because, well, rawhide development branch lolz...). In Gnome 3.16 on Wayland, you can't really expect to have a stable system if you're using non-open-source graphics drivers, and there are some pretty weird things, like in games that were ported to linux/X11, the mouse goes haywire, or in nautilus, sometimes things don't work as you'd expect them to work, and some other things.
Wait for Fedora 22. You can use testing, but you might lose rmpfusion and the rest as they dont sync up with releases the same time Fedora testing versions change.
Shit you reminded me that Non Free repo is not up for 22 yet. So you might have to wait for like 30-40 days until Fedora 22 has full support from all major parties.