Offsets of MPU
Every MPU9150 Unit has different Offsets. This means that every time when the MPU chip of the Quadrocopter needs to be replaced it is necessary to calculate the new Offsets of this chip.
To do this, theres is a sketch in the repository named "MPU6050_calibration". This sketch was written to work with the MPU6050 but it works fine with the 9150 as well.
After you placed the new MPU9150 chip on the copter and added the wireing you need to bring the Quadcopter in a balanced position. Use a water level on the top of the motors to check this.
As soon as the copter is leveled, you upload the calibration sketch to the arduino and start the serial terminal.
Just follow the instructions the sketch is giving to you and after it finished it gives all six offsets as outputs. Integrate these Offsets into the MPUHandler.ino code and you are clear to use the new MPU9150 chip.