How to change the Pattern/Design of my header

Forgive me, I'm a newb... I can see in my 'patterns' page that there are different pattern options for my header. I want to use a specific one that has the logo at the right size and a burger menu (on desktop). How the heck do I change the pattern of my header to the one that I want?

I've seen a whole bunch of articles and videos that tell me to go from appearance to editor, but my dashboard doesn't have that option.

If I go onto customise, it only lets me change the navigation and the 'site logo' but not the size of it.

Please help!

Edit: The theme I'm using is Barnsbury