According to the theme developer’s site () that is a feature available only to those that donate to them.
Anyways when it comes to free themes the best practice is to just leave the theme copyright footer as it is because that is the least you can do to help the theme developers that created the theme and released it for free.
But hey you can send them an email and see if they can help you with that.9 times out of 10 they will help you with your problem even if you aren’t a donator. The best people to help you with theme problems are the ones that coded it.