1.To customize header, go to WordPress Menu > Appearance > Arcade Themes Setting > Header
2. You will see on your left side, you can change the logo and also header background image.
3. You can see that you have to install jetpack plugin in order to add logo.
4. You must add the image with size of 1950 x 500 pixels for the header background image. Click add image and choose the image that you want. It will change immediately.
5. Besides that, you also can change the text and link color.