Additional possibilities:

You can quickly and easily change the hierarchy level of a category by drag & drop - drag a category to the desired position and release the mouse button to drop it there.

If you need to change the category of selected tasks, drag and drop them onto the needed category.

Right-click on a category to call the context menu with the full list of actions applicable to that category.

You can easily change the category icon - choose Edit Category in the context menu and change Image in the Edit Category dialogue box.

To expand or collapse categories with subcategories choose Expand All or Collapse All in the context menu.