“This guide describes the installation of Fairphone Open from the stock Fairphone OS. If you switch to Fairphone Open, all your data (including apps, messages and pictures) will be erased.”
There are more options for installing Fairphone Open OS available at the download page …
You can install the “Upgrade from an earlier Fairphone Open build.” OTA file using the TWRP recovery of Fairphone Open OS.
You can actually use the “Upgrade Fairphone Open using a script.” manual file instead of the switcher file with the steps of the Installation guide.
You can switch from Fairphone OS to Fairphone Open OS, losing all your data, with the “Install Fairphone Open on a device running Fairphone OS.” switcher file.
@Douwe … Perhaps the Installation guide should cover both the switching as well as the updating/reinstalling scenario? The steps are the same, only the files to be used have to be made clear.