In uniform circular motion a particle completes a revolution in a fixed time and at every point of its motion the rate of change of distance w.r.to time is constant.So its speed,a scalar quantity remains unchanged.
But in circular motion the direction of motion continually changes with time, so its velocity which is a vector quantity changes with time due to change in direction. Hence linear momentum which is a product of mass and linear momentum also changes with time.
But in uniform circular motion the angular velocity
Hence its angular moentum