CRS2000 How to SET THE REGISTER NUMBER

I've been through every Menu I can find, and tried numerous things. The CRS3000 Manual is no use for this.

Does anyone know how to set the Register Number, one of ours has lost it's number and now won't fit back into the system we have of six (the other five are fine).

If any body has any suggestions of how to set it again, I would be extremely grateful.

Regards
Graeme