If you want to change the template to your existing items, you need to select them in monitor page (checkbox left to the titles), click BULK OPERATIONS button:
In the window which would open, check TEMPLATE checkbox, choose the template to which you want to change and click "APPLY":
This action would reload all your item descriptions from Amazon.
Title, images and item specifics would be left as they were untouched.
*The action is not supported for multi-variation or bundle listings and neither for any type of listings from AliExpress.