Region Change (Dreamcast)
The region information of a Dreamcast is stored in the Flash Mask ROM (IC502).
In order to reprogram the Flash ROM you need to burn a disc with the Dreamcast Region Changer image (using ImgBurn with the Dreamcast Drivers), or place the image on a GDEmu to run it.
Allowing Flash Write
In order to enable writing of the Flash Mask ROM, you need to pull R512 resistor to the 12V rail.
Insert the Dreamcast Region Changer CD (or run the image from GDEmu).
It is as simple as selecting the region, format and language, then selecting Write.
To confirm it worked, click Read and check it does not update back to the old region.
Once done, click Restart, then turn off the console and the remove the R512 wire. Leaving this on could accidentally re-write the Flash ROM, or overheat it according to some reports.