The Types Editor interface is a simple two-list selector as shown in the screenshot below.
Tab Bar with Types Editor displayed
The list on the right shows the current topic type(s), the list on the left shows the other possible topic types allowed by the schema. To add another type to the topic, select the type from the list on the left and click the Add Type button. To remove a type from a topic, select the type from the list on the right and click the Remove Type button.
Note that changing the type of a topic alters what properties and associations it is allowed to have, however this will not be reflected in the editor interface until after you save the topic.