Sure thing! Navigate to Appearance > Editor, and double click the header to edit the header template part. Select the Group Block for the header (this is easiest using the list view option). Under the “Advanced” tab for the Group Block you should see an “Additional Classes” field. Remove the “position-fixed” class (leaving “header-wrapper”). Then save the changes. That should create a solid white header bar at the top of the site.
I attached a screenshot for reference. I hope this helps.