Question about child theme edition (beginner)

Hi!

I’m a beginner on websites edition, so I guess my question might be easy to be clarified.

I’ve just activated the child theme on my website. The theme is Emphaino.
I’ve already done the files “style.css” and “functions.php”, activated the theme and everything is working correctly.

However, I still haven’t made any changes regarding the original theme and that’s where my question resides: From now, how should I do to edit my website?
I thought I was going to open the Editor on the Control Panel and the full code would be there for me to edit it, but the only code that appeared was the one I uploadead (style.css).
For example: I want to change the menu position and put it under the header. I used “Inspect element” Chrome’s tool and already identified the part related to the menu, so I believe it would be just a matter of cutting this part of the code and pasting it below the header code, however, I don’t know how to proceed, where I should go to make this change…

Anyway, I apologize if I’m saying to much bullshit (or if I didn’t express myself correctly cause I’m not an English native), however, as said I’m a beginner. I’d basically like to know how to edit my website from now on.

Thanks a lot!