We know that L=Iω That is Angular momentum is equal to product of moment of inertia and angular velocity.
Now differentiating with respect to time,
dL/dt=d(Iω)/dt
But we know that the rate of change of angular momentum is equal to net external
torque i.e.
dL/dt=τ
therefore if no exterannal torque is acting on the body i.e.
τ=0then dL/dt=0
in other words d(Iω)/dt=0
because
dL/dt=d(Iω)/dt
Hence angular momentum is conserved or angular momentun does not change with time if the net external force acting on the body is zero.
hope this helps