What is the law of inertia for rotating systems in terms of angular momentum?

The law of inertia states that if an object is in motion will remain in motion unless an external force is applied to it.

law of inertia for the rotating system in terms of angular momentum states that the angular momentum of a system will remain the same unless an external torque is applied to the system.

External Torque either increases or decreases the angular velocity thus changing angular momentum.