With the Divi theme installed, the Theme Customizer provides a range of options for configuring the way Divi looks. One set of options in the customizer is the "Header & Navigation" options for configuring the way the Divi header bar looks. If you find that these options are missing or appear briefly and then disappear, here's what to do about it.
First, here's an example showing the header & navigation options where they normally appear:
In order to make changes to the header you need to instead go into the theme builder ("WP Dashboard > Divi > Theme Builder"), locate the relevant header layout and make the changes to it there.