Can my husband and I change our prenup?

We signed a prenuptial agreement before our wedding and we’ve been married 10 years now. When we signed the prenup, we each wanted to keep the property we had before we married. I’ve recently started a business and my husband and I agree that if we were to divorce, I get to keep the business. He isn’t working in it and isn’t putting any money into it. So now we want to add that clause to the prenup. Can we just amend the prenup ourselves so he doesn’t have an interest in my business or do we need a lawyer to do it?