I could be wrong, but to the best of my knowledge, you can't change the names of the predefined fields. If you want to have your own categories, use one of the Custom fields (Define menu -> Custom Fields).

Dave Sawyer
iMac 24 2.8GHz Mac OS X 10.6.2
PowerMac G5 2GHz DP Mac OS X 10.5.8
Windows XP, Vista and Windows 7