Well, I'm trying to avoid doing a tutorial on how to use Linux, or raspberry pi's. There are much better tutorials out there for that.
There are a thousand ways to edit files on the command line in linux. I use vi, but you should use probably use nano:
Also, you don't need to be root (linux admin) to edit the config.yaml file. It is owned by the pi user. So you should just ssh in as the user 'pi'.
copy the file first, so you can revert if you do something wrong:
cp .octopi/config.yaml .octopi/config.yaml.orig