How to install a new LineageOS on your old Android device:
==========================================================
Summary of the procedure: (working with a Linux computer)
- use JOdin and Heimdall to install TWRP on the phone
- use TWRP to install LineageOS
- no rooting required
==========================================================
The detailed procedure is as follows:
(In this example, we will install LineageOS 17.1, which is based on Android 10, on a Samsung Galaxy J5 phone from 2015, for which Samsung online provides Android 6.)

------------ on the phone ------------
1) Enable developer options following the procedure described on the following page:
https://wccftech.com/enable-developer-options-in-android-6-marshmallow/

2) Go to "Developer Options" and enable "OEM unlock" and "USB debugging", disable "verify apps via USB".

3) Enable "Unknown sources" from "Lock screen and security" settings

4) Connect phone by USB cable to computer

5) Enable Downloading Mode as follows: Connect your phone by USB cabel to computer. switch off your Samsung Galaxy J5 device then Press and hold *volume down key*, *home key* and *power button* at the same time for 15 seconds, on the next screen confirm with *volume up* key.
(procedure: https://www.droidthunder.com/how-to-boot-galaxy-j5-prime-into-download-mode/)

------------ on the linux computer -----------------

6) Download JOdin3 from
    https://odinflashtool.com/downloads/download-jodin3/
if necessary install the Java Runtime Environment from your software center and check that JOdin starts correctly

7) install Heimdall and Heimdall-frontend from your software center or by hitting "Install Drivers" in JOdin

8) run "sudo Heimdall-frontend" from the terminal

9) In Heimdall, go to "utilities"
    - click detect device
    - use download PIT: save as & download

4) Start JOdin3 on your computer, wait for the green "connected" light under ID:COM.

4) load the pit file you saved with Heimdall-Frontend by clicking PIT in JOdin; disable "Auto reboot" in JOdin

5) Download with the computer the newest TWRP .tar file for your phone, in my case from https://twrp.me/samsung/j5nlte.html

7) in JOdin, click "PDA" and open the TWRP tar file. Click "Start" and wait for green "pass" in the upper left corner.

----------- back to the phone -----------------
10) press "vol down + home + power" to reboot, and immediately:

8) when the phone reboots, quickly switch to holding "Volume UP + Home + Power" to go directly to recovery mode without starting the old android [you have to be fast here; if the old Android boots it will undo your TWRP installation automatically]

9) Use TWRP for Advanced wipe of the following folders: System, Data, Cache

-------------- back to the computer ------------------------
10) on the computer: download from https://sourceforge.net/projects/galaxy-msm8916/files/lineage-17.1-20200826-NIGHTLY-j5nlte/ the file "lineage-17.1-20200826-NIGHTLY-j5nlte.zip"
(see also https://forum.xda-developers.com/t/unofficial-beta-q-rom-lineageos-17-1-beta-for-galaxy-j5-j500h-fn-f-g-m-y-8.4038777/)

10) and download from https://opengapps.org/ the android 10.0 ARM version of Gapps (e.g. nano)

11) select "Transferring images via USB" on the phone so that it becomes visible in your linux file manager, and copy the two downloaded files to the phone

--------------- back to the phone -----------------
12) boot to recovery again and use TWRP to install both zips without rebooting in between

13) you're done, you can now click reboot in TWRP and enjoy your new LineageOS