How do I add/change columns in CiviReports?
The short answer is "You can't unless you're a programmer".  This is a feature that was never added to CiviReport.  However, you have many alternatives.


You can use profiles to create a Search View and use Advanced Search instead of CiviReport.
You can see if the Extended Reports extension has a report that matches your needs.
If the field(s) you want to add are custom fields, you may just need to set them as searchable.
You can hook up a third-party reporting tool.
Drupal and Backdrop users can use CiviCRM Views Integration to get the exact columns/order they want.
If you ARE a programmer, there are several questions on Stack Exchange to help you with that, along with the CiviCRM Report Reference in the Developer Guide.