Want to edit header but facing issues

Hello, I’m not a professional developer, but I do have some knowledge in this area. I’ve created a website, and I’m currently facing an issue with changing the header. When I make changes to the header and update it, these changes apply to the entire website. I’d like to know how I can change only the header on my website. Could someone please provide guidance? If needed, I can also share the website link. Thank you.

What platform are you using? For example, Wordpress, Wix, SquareSpace