Why is the motion of an athlete moving along the circular path with Constant speed considered to be an accelerated motion?

The speed is changing its direction all the time. There is an acceleration which changes the direction of the speed - that is called centripetal acceleration. Only uniform linear motions are considered to have no acceleration.

This is the general formula for acceleration

a = dv/dt

When calculating dv, you should keep in mind the change in the velocity vector’s direction. You can easily see in a graph that with dt tending to 0 (so the length of the arc covered is also tending to 0), the difference between vectors Vf and V0 has a direction which is perpendicular to velocity (the shorter the arc, the closest the angle is to 90 degrees).

There is a formula (which can be deducted from the previous formula) which allows you to calculate the acceleration:

a = v^2/r

Let’s talk about the units:

v is in m/s

r is in m

so v^2/r

is in (m/s) ^2/m = (m^2/s^2) / m = m/s^2

which is the same unit as dv/dt:

dv/dt = (m/s) / s = m/s^2