In the Data tab, next to each Structure name there is a menu button to handle the default templates for a type.

There is also another way of achieving the same result. You can go to the resources, select the desired DataTemplate and in the properties panel select the appropriate type for the DataType property. Then you can double click its resource name and delete it, so it will use the DataType as the key for the resource.
As this process is non-intuitive at all we're looking for alternatives, for example by making more explicity the conversion of a DataTemplate between a default one vs a named resource using a toggle in the resource.