const
   n = 4;
type
  TMx = array [1..n,1..n] of integer;

// ...

procedure Mirror(var mx : TMx);
var
   i,j : byte;
   T : integer;
begin
   for i := 2 to n do
    for j := 1 to i - 1 do begin
       T := mx[i,j];
       mx[i,j] := mx[j,i];
       mx[j,i] := T;
    end;
end;

If (N mod 2)=0 then
R:=(j div 2)
else
R:=((j div 2)+1);

If (N mod 2)=0 then R:=(j div 2)
else R:=((j div 2)+1);

  for i := 1 to n do begin { по строкам }
    for j := 1 to n div 2 do begin { по столбцам }
      T := arr[i, j];
      arr[i, j] := arr[i, n - j + 1];
      arr[i, n - j + 1] := T;
    end;
  end;