Actually I haven't found the copyright code yet. But I have changed the 'Forum software made by Xenforo', this line and I have changed the copyright portion of the theme I used, though it was also took some time to figure out.
Changing the line, 'Forum software made by Xenforo' is easy. Enter the ACP. Then go to Setup > Options > Basic board information > Board Meta Description and change the line. The metadata will be changed automatically.
Now, I have used Vertiforo theme. I changed the copyright portion in the source .xml file before uploading. If you open the .xml (I use Dreamweaver to open .xml files) file, you will find the 'Footer' portion from line 580 to 587. You will understand what to change there. That's all.
But I am still looking for the original Xenforo copyright entry.