I can't change the logo

Please see this screenshot: https://i.imgur.com/UZAttpD.png ... I am trying to change the current logo to the black-and-white one shown in thumbnail on the left, but it just won't change no matter how much I press "Change Logo".

Also, it says the image should be 350 x 70 but a) that's tiny and b) the header as shown here, and in another site I'm using GP on, is more than that, IIRC about 750 x 200 or similar.

Also, do you have any step by step tutorials? The documentation is very confusing as it assumes so much knowledge I don't have. Such as, what is the difference between a header and a logo in this context? And what does parallax mean and how does it work? And what is the difference between what I find on the 'customise' section and the GeneratePress section - when do I use one, and when the other?

I try to follow this: https://docs.generatepress.com/article/merging-header-navigation-content/#create-a-page-header ... and it's confusing entirely. It says that Page Header has been replaced by the Elements Module. So this page is defunct? But yet it's still there. And I can't find the Page Header option.

I have been trying to figure this out for days upon days and it's getting so frustrating, so your help would be appreciated.

Tracy Culleton