I have not used Preferences, so I had to look at the .h file. It appears to be 'keyed' data. I assume the MAC is the key. Since the API does not contain an update, it would appear you need to do a remove by key, followed by a putString, perhaps?
Sorry, I will have to drop out, I would never use this functionality since I wrote my own many years ago so I am sort of blind to the preferences API.
Good luck.