To apply a Task Template to an item that has already been created, all you need to do is update the Task Template field on the item. For example, I'll update the Task Template field on the Blue Jay asset with the Main Character template. Once I've made the change, Shotgun will take care of creating the tasks with the template information. Easy peasy.
non-destructive and additive by nature. This means that any existing Tasks won't be deleted or updated in any manner; only new tasks will be created. Should you need to remove old tasks or update existing ones, I'd suggest using filtering and our power editing tools.
Changing from one Task Template to another is also non-destructive, so if you need to switch an item to a completely different Task Template, you should first delete the existing Tasks on that item (Select them all > right-click > Delete Tasks). Then you can clear out the Task Template field and select the alternate template you'd prefer. Once the new template is applied, new tasks will be created and linked to that item.
I covered a lot, and some of this stuff can feel a little magical. Should you have any questions or would like to know more, let us know in the comments.