Since you appear to be using the CD/download version of TurboTax, you can make the adjustments directly on Item H of the Dependency/EIC Smart Worksheet of each Dependent and Nondependent Information Worksheet so as to indicate that you, and not the other parent, will be claiming the dependent. Of course this is possible only if the child otherwise qualifies as your dependent.
You might also be able to do this in step-by-step mode by first deleting the dependent, then reentering the dependent and indicating that you have a custody agreement. Simply changing an existing dependent entry to say that you have a custody agreement won't necessarily automatically remove the indication that TurboTax already recorded indicating that the other parent will be claiming the dependent.
