When you've got a Rescue CD up and running, you're in a complete Linux environment. But none of your hard-drives are mounted as a root filesystem, so you can edit them at will. And you've got a lot of tools with which to do it, although they're probably not GUI-graphics programs.
So, as you saw, you create a mount-point directory and mount one of the drives onto it. That'll give you access to, say, the boot partition (probably #1), the regular root-partition (maybe #3), and so on.
be a learning curve to go through; that's unavoidable and it's steep the first time. Maybe the best way to approach it is to shut-down a working-well
system, boot up a rescue-CD, and then (carefully) look around. See what's on it. Much better to do that when you know that you can simply reboot, than to wait until ...
... the point at which, I daresay, every single one of us did wait-until
... to actually stick the CD into a drive and try it out.