Thank you very much AnotherElk.
I have the android version 7.1.2
I entered in TWRP, wanted to install update zip file but found the message : “this operation may install incompatible software and render your device unusable.” and then “no OS installed ! Are you sure to wish to reboot?” I went back and selected all partitions to backup (I had few time and didn’t have seen your other message…)
Even if I found the message "Unable to find partition for path ‘/system’, I found all my configuration without losing any data.
Now as soon as I have some time I’ll follow your instructions to install Fairphone OS. Thank you again