I bought a Fairphone 2 few years ago, and directly installed Fairphone Open. The main reason is I didn't want to be polluted by the Google echosystem. I know that Fairphone Open is based on Android but it does not have Google services pre-installed and it is full open source.
Years has passed and it seems that Fairphone doesn't really support it anymore. Updates comes to be more than rare (stuck with Android 7), another sign is that it's not even available on Fairphone 3 phones... So as I didn't have the intention to replace it I decided to install LineageOS in order to be up to date. (security and applications compatibility).
We will see how to process.
First you have to know that every datas will be erased so it's a good thing to make a backup.
I've used this two softwares to backup my datas.
Off course I've manually saved my own pictures and videos.
Download adb tools, to your Download folder, unzip and open a Windows Command Prompt. Go inside the unzipped folder.
For GNU/Linux, here with Debian :
user@host:~$ sudo apt-get update && sudo apt-get install android-tools-adb android-tools-fastboot
C:\Users\std\Downloads\platform-tools_r30.0.3-windows\platform-tools>adb devices * daemon not running; starting now at tcp:5037 * daemon started successfully List of devices attached f9f7d41f device
In order to install LineageOS we need to replace the default recovery by TWRP.
Download latest TWRP for fairphone 2
adb reboot bootloader
fastboot flash recovery twrp-x.x.x-x-FP2.img
Download latest build for FP2 here.
Optionally, if needed (I personnaly don't), you can download Google apps here.
adb reboot recovery
adb sideload lineage-17.1-xxxxxxxx-nightly-FP2-signed.zip
adb sideload open_gapps-arm64-10.0-xxxx-xxxxxxxx.zip