Let me start by saying that @JImDearden's answer is the correct one. You don't need a regulator for this motor. The answer I'm going to give assumes that you, for some reason, still need a regulator. While this doesn't exactly directly apply to this question, it brings up some things to consider that are worth knowing.
Don't Use A Linear Regulator:
Going from 7.4V to 6V will require a voltage drop of 1.4 volts. At six amps, that works out to be 8.4 watts. This means that your linear regulator will be outputting 8.4 watts of heat. That might not sound like a lot, but when it is concentrated into a small area it is enough to break things. You would certainly need a reasonable heat sink on this, just to get rid of the heat. But more importantly, that 8.4 watts of power will be wasted and since you are running on batteries this is not a good thing.
You Could Use A Switching Regulator, But:
A switching regulator can easily handle the 6 amp output that you require. However, you would want to carefully design it for maximum efficiency. If you don't take care in designing it, it would be about 80% efficient. With 36 watts of output, an 80% efficient switching regulator would waste 7.2 watts-- not significantly better than the linear regulator. With care, a switching regulator can be 90-95% efficient, and only waste 1.8 to 3.6 watts.
Switching regulators are also complex, and is beyond most hobbyist-level EE's. But buying a switcher module is an easy way to do it, without the difficult part of designing a proper PCB.
But PWM or Current Limiting is a good alternative:
Motors rarely care a lot about the voltage, it is total power (and thus, heat) that they care about. (There are some important caveats that I'll cover later.) It is possible, and common, to run motors at a higher voltage than what they are rated for. Two ways to achieve this is by using PWM or current limiting be within the rated power.
Let's say that you run the motor at roughly double the rated voltage. In that event, you can run the motor with a 50% duty cycle. Total power is essentially the same as if you ran it at the rated voltage but with a 100% duty cycle.
Alternatively, you can somewhat ignore voltage but run the motor at the rated current. Many motor driver chips can automatically measure and limit the current to the motor for just this purpose.
Caveats: Of course things are rarely that easy, and there are many factors to consider. I've just given a super quick overview. Here are some things to be aware of... Brushless DC motors (like muffin fans) have IC's in them that often can't handle voltages that are too high. Brushed-DC motors might wear out sooner due to increased arcing on the brushes at higher voltages. A voltage that is very high could cause the insulation to break down, so don't run a 6v motor off of 100 volts. Motors are highly inductive, which could help or hurt you when PWMing. And motor control is a complex subject, and you can make it as simple or as difficult as you want. But the closer to the edge that you push your motors, the more you have to pay attention to the details.