How can I upgrade from elementary OS Freya to Loki without losing my data, you may ask?
There continues to be no officially supported way to upgrade between version of elementary OS. This is due to the installer being a patched Ubuntu/Debian installer with no custom elementary support for upgrades, only detection (which I believe has been upstreamed).
Clean installation is the recommended method, this creates a new working environment and removes any config conflicts commonly encountered when keeping an entire home partition.
Upgrading though the update manager is not even possible, the update managers vary between these versions and this will never be made available. Any upgrade offer you see there is for the Ubuntu backend, not elementary OS.
Upgrade from installation media works much like installing cleanly does, only you leave your data on the disk as well as making a backup. This is untested and results will vary significantly. If you experience any issues, the likely response will be "does this happen on a clean install?"
- Back up everything
- Make some Loki installation media.
- Boot from the install disk and select the "upgrade" option.
Source: Me, I wrote most of the installation guide.
As an Ubuntu, and therefore Debian, variant it should be possible to upgrade elementary OS The Debian Way.
Unfortunately Ubuntu has also shirked their responsibility to use the one true Debian distribution upgrade method. They should never have come up with their "update manager" method, it has only led to laziness and buggy upgrades.
If you familiarize yourself with aptitude, the Debian method is sure to work. It isn't pretty--it may cause packages to be removed that you wish to keep and my leave behind obsolete packages--but it works, and there are solutions to both of those issues you can implement after upgrading.
The concept of having to erase your hard drive and install from scratch for every release of an operating system is a holdover form the Windows 95 era. Ordinary users should not be forced to suffer through this. I used the Debian method in Ubuntu for years; it taught me a lot about the apt package management system that I was grateful to learn and it kept me from having to lose any precious data for about a decade.