Re-using templates without editing
There are a couple of ways to do this.  The way I'm describing below isn't the easiest method, but it should work in all versions of CiviCRM.


Go to Mailings menu » Message Templates.
Press "Edit" next to the template you'd like to copy.
In the editor, you should see a "Source" button.  It may look different depending on your version of CiviCRM, but the most common version looks like this:

You'll see the raw HTML, which looks like gibberish.  You'll want to do a "Select All" and "Copy" (Ctrl-A, then Ctrl-C on Windows).
Press "Cancel" to go back to the message templates selection screen.
Press "Add Message Template" to start a new template
On the new (blank) template, press the "Source" button.
Paste in the raw HTML from the origin template.
Press "Source" again, and it should look like the original - but now you're in a copy of the original template!  Save this one with its own name and edit it however you like.