Simple CSS Question

I’m new to all of this. I just need to know how to edit ANY css in my child theme. I tried to follow some instructions I found, but nothing is changing.

Specifically, I’m trying to get all of the page headers to be centered.

Can one of you guys take a look at my site to see if there any obvious issues? It would be greatly appreciated. Thanks!

My site (don’t judge): https://coopboardgames.com/