This theme has 2 sets of footer copyright text.
a. The theme's copyright "Powered by Mantra and WP"
b. The optional your own custom copyright text.
The a. you can't do nothing without doing thing in PHP file, and no body will recommend you or anyone who are not familiar with PHP to edit template on a live site because it risks making mistake.
The b. you could manage to do it under theme's setting, it's available under the option with a lable "Custom Footer Text". This theme setting is for user to put in text so there's no risk making mistake to take the site down or anything.
The a. is what's bothering you because it's been altered, I think, to the theme file directly, details in my first post.