I want rounded edges only on the top of my header…

Hi, I am completely new to building websites. It is slightly intimidating at the moment, but I am going to enjoy the learning process.

This is the current site I’m working on:
https://smartachprocessing.com/

And I have the weaver theme installed.

What I want to know is how I can get the top edges of my header image rounded, so that the header and the menubar look like one thing. Hopefully that makes sense.