depending on the slot the card is in the command will be a 3xx say if it is in slot 4 the command will be 304 then go to the register number where that resides - say if it is on port 4 then it would be command 304 register 4 - at that point you will need to reenter all the info on the line until you get to the last 3 digits (if they are 3 digit ext numbers) and put in the new extension number then hit save (if you are doing it locally).

I would suggest before you change the register you jot down all the info on that particular register. After you have put in the new ext then go back and check that all is the same except for the new ext. Then just exit out.