Basically, you want a voltage regulator.. something that takes a higher voltage as input and produces a steady lower voltage as output.
The easy way is to use a linear regulator like the LM7805. That takes any input up to about 30v and produces 5v output. The trouble is that the LM7805 is basically a self-tuning resistor, so all the voltage you lose gets wasted as heat. In this case, you'd generate 7W of heat for every 5W that reached your motor.
The efficient solution is more complicated. That involves a 'switching regulator' that basically opens and shuts a switch really fast. It's pretty much the same idea as PWM, just faster, smoothed out better, and with better control of the output.
Some "low dropout" voltage regulators use switching circuits, but it can be hard to tell from the datasheets. You can get dedicated switching controllers and switching regulators, but you usually need to build a circuit around them. If you're going to do that, you might as well look at pre-built circuits like this one:http://www.mouser.com/ProductDetail/Mur ... %2f%2fo%3d
That one costs about $12.50, but will drop your battery voltage to 5v at 2A with a simple drop-in circuit.
When you void a product warranty, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.