Thank you for reaching out to us. In X if you go to X > Theme Options > Header there you can find all the options to customize the header (see screenshot below) but if you need to the changes beyond that then regretfully we cannot support any modifications to the header other than minor cosmetic changes due to the fact that X is a highly dynamic theme with many elements being constructed on the fly based on options in the Customizer. Among these, the header is one of the more complex elements of the theme and structural modifications to it will likely lead to numerous layout problems as there are a lot of things to take into consideration because of X’s responsive nature. Taking all of this into consideration, these types of updates are out of the scope of our support as their involvement is simply too great. Additionally, we will not be able to support any issues that might arise from modifications made to this area on your own.
However if you want more complex changes in header or you want to build a custom header then I’d suggest you to upgrade to Pro, as Pro has header and footer builders which gives you the ability to create your own headers and footers and assign them to different pages of your website. You can control the layout and how different elements which creates the content of the header or footer are positioned. For more details please see https://theme.co/apex/forum/t/the-difference-between-x-pro/39341
Also see https://theme.co/apex/forum/t/pro-header-and-footer-builder-introduction/103
Thank you for your understanding.