A case has been performed and closed in Novari ATC and WTIS but we now need to make an edit. Is there a way to send an interface message from Novari ATC for the closed case with the corrections?  

No. Once a case is closed in Novari ATC, Novari ATC does not send any additional messages to WTIS. WTIS automatically fails any messages on a closed case; therefore, any edits or updates that are needed for a completed case will have to be done manually.  

Please refer to How can I fix case completion Issues? for instructions on how to manually edit a completed case.