You can simply detach the instance using the Console UI. This will remove the instance and create another:
In the dialog, make sure to keep capacity unchecked if you want a replacement instances to be launched.
Draining Timeout - Specify the time to keep the instance active for the before terminating it.
Decrement Group's Capacity - Change the group Instance count Target reducing it by a single instance - This will mean no replacement instance will be launched.