As for the manual install there are topics on that, my next post links to the official Faiphone instructions. Once you have the version you want even /e/ or LOS you will need to install the Android Debug Bridge (ADB) to a computer and use that to access the phone etc.
You can search for (adb) and ensure you are happy with that before doing anything else. i.e. download adb and connect to your phone. Once you are confident you can connect to your phone via computer you can download a version and install/flash to the phone.
Here’s a link to the download options: