Header

Not sure how you “installed” a custom header but the normal method is to do this:

Dashboard >> Appearance >> Header

You should now have the option to upload your header image.

I can’t test this because you are using a premium theme which I don’t use. There are many good free themes from www.ads-software.com and my recommendation is to use Coraline but again it is your blog and your choice of the theme.

Good luck.

Make sure you upload the new image to the correct directory / folder on your webserver. In this case:
1. login
2. click ‘settings’ ‘media’
3. deactivate ‘organise my uploads into month- and year-based folders’
4. make sure your webserver has a folder ‘wp-content/uploads’ if not create one
5.upload the new image to that folder
Check if things work in the browser [press F5 tp refresh the page]
If not:
Open the options page of your theme and check if the path [folder and filename of the new headerimage match that of what you’ve just uploaded. Correct if required.
Check if things work in the browser [press F5 tp refresh the page]
Good luck

what is the code in header.php for the header image?
this might be wrapped in a conditional statement (?)
does the theme demo show header images on sub pages?

you can paste the code into a https://pastebin.com/ and post the link to it here.