Categories and Forums can be deleted using [trash] icon-button located on right side of each forum panel in Dashboard > Forums > Forums admin page.
Forum delete consists of two steps, after clicking on Trash button, you’ll be redirected to second step page. On this page you should select forum content deleting type.
Delete Forum with content
This option also deletes all child forums, topics and replies of the forum you’re going to delete. Be careful with this option. Select it only if you are sure that you want to remove permanently all child forums, topics and replies of this forum.
Delete Forum without content
This option allows you to keep all child forums, topics and posts but you need to attach them to other forums. Once you selected this option, it activates dropdown menu with Forums Tree. Just select the target forum and click on [delete] button. All child forums, topics and replies will be attached to selected forum. Layout will be inherited from this forum.