Are you creating this template in the Template Builder under Assets? If so, all you need to do is click "Create a New Template" and select the template that you're looking to copy. This will create a brand new copy of the other template and you can make whatever changes you need. Of course you'd need to be logged in with your Owner credentials or Manager credentials that have access to all accounts.
Similarly, if you're working IN a sub-account and that account has the "Save As Template" function activated, you can use "Create a New Email" , select the template, make your changes, and then click the arrow beside the SAVE button and choose "Save as template" .
Let me know if you have any questions about this!!