All the new Branded Templates will be automatically visible in the Template ID dropdown. The list is sorted in a new to old order so the new Branded Template should be directly visible on top of the list.
How to test it?
First set up a new editor product with the specific needed settings for the use case. It could be a good idea to duplicate a previously similar Branded Template product.
Then select the right 'Template ID' in the dropdown under the 'Templates'-tab.
Click on 'Save & Edit' and then click in the right top corner on 'Preview' to start testing.