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.

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Thank you so much for this detailed answer! Helps a lot!