A template can be edited, but there are a few steps to take:
Locate the template in the Library, click Use and make your edits.
Send the updated message to yourself.
Once the message is in Tracker, you can add that message to your Library by clicking the 3 little dots, located to the right of the message in Tracker.
Select Add to Library
Name your message so that it is easy for you to recognize and find (this name will only be visible to you). Click save.
You'll now have the original template and your updated template available.
Hope this helps!