To edit the copyright information at the bottom of your template click the Copyright Information tab in your templates parameters. From here you can edit the text in the Copyright field to your own taste.  If you wish to remove the copyright text entirely simply leave this field blank.

Note that this field accepts HTML so if you wish to add a link to your copyright you can use something like the following..

<a href="http://www.joomla51.com">Joomla51</a>