Most aftermarket stereos come with a universal collar that will work in a variety of applications. These collars can often be installed without additional mounting hardware, because they have metal tabs that can be bent out to grip the sides of the dash receptacle.
In this case, the single DIN collar is too small to fit directly into the dash, and it also doesn't fit inside the existing bracket. That means we won't be using it. Instead, we'll simply screw the new head unit into the existing bracket. Note that the existing screws may not be the right size, so you might have to make a trip to the hardware store.