The idea of bracketed shots is to get a range of exposures. So you can...

1) pick the best one
2) layer then and blend them to create one single photo with the best bits of all the exposures.

To to bracketed shots well to blend, you need the camera on a tripod or stable surface when you fire off the shots to ensure that when editing, everything aligns well.

If you can find your manual, you will find that there are a heap of settings you can use for bracketed shots. And my guess is that either the default, or someone set it, is for ISO to remain the same and shutter speed/aperture to alter to get the bracketing range. For me, bracketed shots are done with ISO and Aperture being constant and shutter speed being the variable. So to answer your question, somewhere in the camera menu will be an option related to bracketed shots, and the settings should be in there.