Before you go doing anything, answer a couple of questions: Is the 7.4V battery I'll assume it's a LiPo) driving the car? Is it connected to an electronic speed control (ESC)? If the answer to both is yes, then you don't really need to do anything as the ESC will drop the voltage down for the receiver and in turn, your servo.

Depending on what ESC you have will depend on what voltage and current it puts out. Basic models will only do 5V @ 1A, the better ones will do 5.5 - 6V @ 3A.

I guess the issue is if your servo can run off the lower voltage. Most will, but will not be as fast as they are at 6V.