I'd be happy to help you out with this. For most themes, you should be able to disable the "Enable sticky header" setting. Head to your Online Store > Themes > Customize > click on the "Header" section on the left hand menu.
From here, you can simply uncheck the "Enable sticky header" setting.
If you are not seeing this setting, please let me know which theme you are using and I can try to locate the setting for you.
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