Hello @janansiam,

To change the header in category page add below given CSS by going to Admin Panel > Appearance > Customize > Additional CSS.

body[class*="category"] #banner-image {
	background-repeat: no-repeat;
	background-position: center;
	background-size: cover;
	padding-bottom: 55%;
}

body[class*="category"] #banner-image img {
	display: none;
}

.category-73 #banner-image {
	background-image: url('https://cms-assets.tutsplus.com/uploads/users/1500/posts/31260/image/summer_nature_background_with_green_grass_and_butterfly_and_blue_sky.jpg');
}

For above given CSS, you have to replace the category ID with your category ID. Similarly, for other category add the same CSS which is :

.category-73 #banner-image {
	background-image: url('https://cms-assets.tutsplus.com/uploads/users/1500/posts/31260/image/summer_nature_background_with_green_grass_and_butterfly_and_blue_sky.jpg');
} 

But with replacing the category ID.

You can find the category ID either by inspecting or from the backend.

Screenshot:

https://prnt.sc/lswsym

Hope this helps.

Regards!!