My understanding is that you can't change the bias on
most consumer grade CCD cameras, it is set in the factory.
To remove the bias all you need to do is subtract your dark frame, the bias will be included in that subtraction. If you want to scale a dark frame or remove the bias from a flat frame then you will need to take a dark image of zero time length (or as close as you can get). Your software with the camera should allow you to do a zero time exposure. You can then subtract the master bias (remember to take a few of them and median combine them
) from whichever frame you want to.
For general purposes though subtracting the dark from your light and subtracting a flat dark (if your flats are more than a couple of seconds long) from your flats prior to dividing them into the lights, will deal with any bias in your images.