How to remove an app from app lab - and how to replace it with another app

We need your help! Our old app is no longer under production and is no longer for sale, and must be removed from app lab.  We are no longer in contact with the devs who coded this app and have a brand new team of devs. The new team has created a much better app with a ground-breaking technology that we wish to add to app lab. What are the steps we need to take as the owner-producers? Many thanks for any and all help