A body tends to be at rest or in state of uniform motion unless and until an external force acts on it.
Hence if a body is in a state of uniform motion it continues to be in state of uniform motion
as external force is 0 .Hence the body has uniform velocity , therefore displacement changes with time.
Using second law of mtion.
By formula F= M*a ( "F" is the force , "M" is the mass of the body ,"a" is the acc)
SO according to question F=0, Hence a=0.
Let us assume that the body has initial velocity "U"
Then by kinematics we can find the displacement
(ie) s=U*t + a*t*t/2 (a=0)
Hence s=U*t (this indicates that displacement changes with time).
You also may be in confusion as in real life this is not possible ,
this is due to an external force "friction" due to which the motion of the body comes to rest.