There is no obvious way to modify the copyright information that appears in the footer area of a Joomla template. You can set Enabled to off for the Footer module (mod_footer) in the Module Manager, which will remove the copyright information.
Editing the Joomla Template
If you would like to modify the copyright information, one method is to edit the index.php file for the template. Find the footer code in the template, which will be similar to
<jdoc:include type="modules" name="footer" />
You can delete this code and enter your own copyright information. The copyright symbol must be entered as © instead of cutting and pasting the copyright symbol from a font file.
Modifying the Footer Language File
A better way to modify the copyright information, which will work across any Joomla template, is to modify en-GB.mod_footer.ini file found in the languages/en-GB folder. The file and folder name are language dependent, so they will be named different from languages other than English.
The section to modify is the line with
FOOTER_LINE1=Copyright © %date% %sitename%. All Rights Reserved.
Copy this line and comment out the original by putting a # character at its beginning. I modified this line by placing the starting year of my company in it, i.e.
FOOTER_LINE1=Copyright © 1998-%date% %sitename%. All Rights Reserved.
You only have 2 footer lines to modify text. If you need to modify more, then you should modify the Joomla template directly, or create a mod_custom module to appear in the footer module.