Use a Encrypter-decrypter software.
Decrypt the itemtype.dat, make the edits you wish to make and then encrypt it.
Here is a link to itemtype.dat guide:
The attachment is what I use(I usually don't brother 'cause of the time it takes, would be nice to build an algorithm that does it for me....Or if someone knows some helpful MS Word functions, tell me
). It was made by some forum member whose name escapes me at the moment.