How to make the header take entire of the screen?

Do you mean even if the content remains fixed width, you want the header to expand width to whatever screen resolution the online visitor has? Without being able to look at site’s source code/CSS, can’t give definitive answer to your question.

Your header (and the rest of your content) are probably wrapped in another container, which does have margins… but as mercime said, I can’t be sure without looking at the actual site.