Hello. The BTT Octopus will be happiest with the newest flavor of Marlin. Which is currently 2.0.9.1 and you can download it from github right here: Download | Marlin Firmware (marlinfw.org)

For setting up the board I have found 2 tutorials right here: OCTOPUS USER GUIDE (niceshops.com)

And here: BTT Octopus Setup Guide - UART, BLTOUCH - Make 'N' Print (makenprint.uk)

Done right when you power on your Octopus you should see this on your display at bootup..