How do I change the telephone type from a 3901 to a 3902 on CS1000/Meridian?

The 3900 series of telephones uses a default for the keys according to the telephone type. 

When you add a new extension, TYPE = 39XX automatically sets the keys. To make sure they are in the correct position you will need to OUT the existing one and create it as a NEW TN with the correct type of telephone.