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.

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.

On top of a region free BIOS, you also need to locate the test pad labelled R422 and bridge it. This tells the Dreamcast to listen to the games region (50/60Hz) and switch to that frequency.

If you do not bridge this, some games will run shifted down on the screen or black and white.

The pad is located near the AV port on the back.