Usually you only have 5 changes to change the region of your DVD drive. If you have used them all, the only way to change again is to let the vendor reset the change counter for your drive and reducing the number of changes the manufacturer is allowed to issue by one.
Unfortunately most vendors do not actively support this feature meaning that you can only change the counter 5 times and that's it. Here's where the NEC RPC2 Reset tool can be used. Once you have no region code changes left, you are able to reset this counter again.
For some hardware dependant reasons it is still recommended to use a modified RPC1 firmware instead.