Header block is a custom post type, where you can create whatever content you want with Visual Composer. Below you will find clues, which I believe help you to edit it:
1. Make sure, that Visual Composer in enabled for header blocks via Settings > Visual Composer

2. To edit block, please go to Header Blocks

and click on Edit).

3. Header block from our demo is created out of two elements: slider

and tabs

4. If you would like to edit Slider – please check the slider settings below:

5. If you would like to edit tabs – please refer to the following tabs settings

6. Don’t forget to save changes with Update button.