In the GAIN tab of the BASIC section in the Assistant.
The second row which is the Attitude Gain is how the Phantom will respond to your stick movement,
in this case only the Pitch and Roll can be adjust. The lower the number is what you want so that it is less sensitive to the stick. Try lower the number 10% at a time. If you have the 7th channel control lever on the back, change from INH to X1, so that you can change the setting at the field to your liking, otherwise you have to hook your Phantom to your computer, set the number, unhook and go fly and come back to repeat the steps until you're happy which will take a lot longer.
Note that you can not set the Yaw and Vertical because Yaw is already slow to the stick movement because it uses counter torque from the opposite motors to do so and the vertical depends on the throttle stick.