You can find a stock ROM for your Galaxy S6 Edge variant here.
https://www.sammobile.com/firmwares/
For custom ROMs, I think Lineage OS supports that phone.
https://lineageos.org

 

also to add to @mikedt you can look for custom roms over at xda:
https://forum.xda-developers.com/f/galaxy-s6-android-development.4010/
your phone is pretty old so i do not know if the links there will still work or not.