Eugene's observations

Having updated an 8.0 system three times I have the following thoughts.  

This is not a service pack or regular Windows update - it is a new installation; at the end you will be left with a C:\Windows.old folder    

I have a faster than average computer and a download speed of about 4.5 Mbps - for me the process took two and a half hours after I finished backing up.

It is always wise to take a backup before you make any change, especially one as major as this.  If you absolutely can't take an image of your entire disk drive, then an absolute minimum must be all documents/photos/music/address books/emails etc. that you would hate to lose.  I didn't have a problem with any of my three upgrades but I did a lot of background reading before I started and some people do run into problems.

The 3.5 GB download is only available from the Windows Store App (accessed from the Modern UI).  It may take a couple of hours to download before asking questions.  There were just 3 questions for me:

1. Did I accept their terms - I did 
2. Did I want to customise settings - no, I was happy with the express suggestions
3. Create a new account - as I always create a local account rather than sign up for a Microsoft one I had to avoid the cajoling into creating a new account and losing easy access to my files and settings.  This is the only potentially tricky part of the installation as the first step is counter-intuitive.  Click on the "Create a new account" link then on the next screen click on "Continue using my existing account".

After the installation finished I had to reboot to get full Internet security protection back.  Then I had to download and reinstall 8GadgetPack to get my clock, clipboard history and Twitter feed back.  I also had to repin FreeCommander to my taskbar.  Finally, I had to log a call with Dymo as their printer driver does not work with Windows 8.1 - their resolution has not fixed the problem either, the case is ongoing.

Reference: Updating to Windows 8.1 from Windows 8