I would not change the fan.
There are two reasons for this...
First, you'll void whatever warranties you have.
Second, the designers chose the particular fan they did because of it's airflow characteristics. Replacing the fan based on other factors such as noise (Note that noise and airflow are usually mutually exclusive) will result in a fan that does not sufficiently cool the fixture.
It's entirely possible that you can get away with it, but you are taking a risk.
If you do decide to proceed, there are a few things you have to do. You should be able to find info stamped on the "exhaust" side of the fan. You'll almost certainly find voltage and current requirements on there. You can use a caliper to find the diameter and thickness of it. You'll likely need to find enough info - such as part number - to look up the airflow information on it - if you can find it.
1. Match the diameter and thickness of the fan.
2. Match the voltage, and choose a new fan that consumes an equal or less amount of current.
3. Match the airflow or get one that is slightly larger (but not a whole lot larger).
That should be it. If you can do that and find one that is much quieter, you can simply swap it in.
