On a Mac, they are stored in a normally inaccessible folder. Here is how you find that folder quickly:

Step 1: Click File/New (it doesn't matter what template this opens).
Step 2: Click File/Save as Template.
Step 3: In the dialog box that appears, enter any word you like in the "Name" space. Doesn't matter.
Step 4: Now look at the "File Name" space below it. There is a magnifying glass icon to its right. Click on it.
Step 5: The folder with all your saved customized templates appears.
Step 6: From this folder, you can drag old templates to the trash, add new ones, or do as you please.

It's that simple.