You can flat out ask for it. If you're at 3Mbps the increase in speed is free since it's already built into your plan. In many cases Verizon will have to validate that the equipment you are on is ready for more than 3Mbps, and then qualify you for the speed. It can take a while, but if you need some ammunition it wouldn't hurt to find out the kind of DSLAM you are on first.
Ask around in the Verizon Direct forum for all of this. »
Verizon Direct