That’s pretty easy:
1. Download Box2d 2.3.0 from the official site, blah-blah-blah, that’s obvious.
2. In cocos2d-x/external/Box2D replace all box2d files with the new ones.
3. If you are using Android platform, add ’Dynamics/Joints/b2MotorJoint.cpp to LOCAL_SRC_FILES variable in cocos2d-x/external/Box2D/Android.mk.
4. If you are using MSVC, add Dynamics/Joints/b2MotorJoint.cpp and Dynamics/Joints/b2MotorJoint.h to libBox2D project.
5. If you are using GCC 4.3*, add ’#include to the following headers:
This is necessary because GCC 4.3* stopped including and some other standard libs automatically to lower the build time. Don’t know why Erin Catto doesn’t update his code accordingly to this compiler behaviour.