First of all, if you are really worried about losing data, you should be backing it up regardless of doing an update or not.
While the 8.1 update is quite large, it does work for most people. If you have an under 1 year old computer, you can check with your computer vendor about the upgrade.
I did it without any problems. I haven't heard of anyone losing data doing the upgrade. Depending on your internet connection, it can take a while. For most I believe it is under 1 hour. Myself, I wouldn't try the upgrade using a CG internet.