The simplest way I've found requires two garmin connect user accounts. So create an alter-ego account (they're free, why not) or get a friend to help. Then make sure the course you want to copy is marked public, open it on the account that DOES NOT OWN IT. Then you get a save-as button. Save it with some new name. Then open the new course on the original account. The easiest way is to copy the URL while the course is opened in the alternate account, log off and login to the primary account, paste the URL. Now you get the save-as button again. Save it, now you have two copies. Now you can edit the copy without affecting the original.
