Update from Fedora 18 to 20 or 21 in few simple clicks and without (I hope) problems!

Sometimes simple things become hard problems. This happened to me when I discovered that my Fedora 18 OS was obsolete (very old indeed) and I tried to upgrade it to the Fedora 20 version. Fedora 18 doesn’t support the automatic OS upgrade so I had to spend some hours to find information on Internet and create the right DIY solution for my case. At the end of my little (sic!) research I discovered that I was able not only able to upgrade the PC to Fedora 20 but I also found a good “trick” to upgrade to the latest Fedora 21 following in just a couple of steps with the help of fedup and some other escamotages. This is how I succeed…