In a circular orbit the speed of the orbiting body does not change because the acceleration vector is always perpendicular to the velocity vector - so the direction of the velocity vector changes over time but its magnitude is constant.

In an elliptical orbit the acceleration vector is generally not perpendicular to the velocity vector (except at the asides of the orbit - the nearest and furthest points from the primary). Remember the primary is not at the centre of the ellipse but is at one of its focal points. So the acceleration vector changes the magnitude of the velocity vector as well as its direction, and the speed of the orbiting body is not constant.

In a circular orbit the speed of the orbiting body does not change because the acceleration vector is always perpendicular to the velocity vector - so the direction of the velocity vector changes over time but its magnitude is constant.

In an elliptical orbit the acceleration vector is generally not perpendicular to the velocity vector (except at the apsides of the orbit - the nearest and furthest points from the primary). Remember the primary is not at the centre of the ellipse but is at one of its focal points. So the acceleration vector changes the magnitude of the velocity vector as well as its direction, and the speed of the orbiting body is not constant.