If they followed the standard instructions, it should be inside /var/discourse. It’s run inside a technology called Docker. Here are the instructions, by the way:
https://github.com/discourse/discourse/blob/master/docs/INSTALL-digital-ocean.md
You’ll need to use some kind of client that lets you access the VPS via the SSH protocol. You’ll have a lot more options of Linux, including being able to use the graphical file manager to edit files on your VPS.