The body moves in a straight line and evenly. Does this change its speed? What is the acceleration of the body?

1) The speed of any arbitrary body with uniform motion will always be a constant value, since for equal intervals of time it will pass the same path segments: V = const = Sa / ta = Sb / tb = … = Sn / tn.
2) The acceleration of such a body will be equal to zero, since it determines the rate of change of its speed: a = dV / dt. Therefore, at V = const, the change in speed is dV = 0 and a = 0 / dt = 0.