KeyMap works on an Association. You have a List of Associations. That is why I Mapped the KeyMap function over the list of Associations.

Regards.

Sir. I don't know why the following error message appears during execution!

    n = 10;
    assoc = Flatten[#] & /@ 
       Transpose[{Range[n], 
         Transpose[{Table[{2019, 10}, {i, 1, n}], 
           RandomChoice[Range[31], n], Table[{0, 0, 0}, {i, 1, n}]}], 
         RandomChoice[Range[100], n]}];
    assoc = AssociationThread[{"id", "y", "m", "d", "h", "m", "s", 
           "Value"} -> #] & /@ assoc;
    assoc[[All, "y"]] = 
      DateString@DateList[#] & /@ 
       Values@Query[All, {"y", "m", "d", "h", "m", "s"}]@assoc;
    assoc = KeyDrop[assoc, {"m", "d", "h", "m", "s"}];
    (*Provides by Hans Milton*)
    assoc = KeyMap[# /. "y" -> "date" &, assoc] // TableForm

KeyMap::invak: The argument {<|id->1,y->Fri 31 Jul 2020 00:00:00,Value->81|>,<|id->2,y->Fri 30 Apr 2021 00:00:00,Value->43|>,<|id->3,y->Mon 31 Dec 2018 00:00:00,Value->11|>,<|id->4,y->Wed 31 Mar 2021 00:00:00,Value->79|>,<|id->5,y->Tue 30 Apr 2019 00:00:00,Value->1|>,<|id->6,y->Sun 31 Mar 2019 00:00:00,Value->44|>,<|id->7,y->Fri 31 Jul 2020 00:00:00,Value->11|>,<|id->8,y->Mon 31 May 2021 00:00:00,Value->27|>,<|id->9,y->Sat 31 Oct 2020 00:00:00,Value->38|>,<|id->10,y->Thu 30 Apr 2020 00:00:00,Value->9|>} is not a valid Association.

or this:

Map[KeyMap[ReplaceAll["y" -> "Date"], #] &, assoc]

Keep the association and KeyMap the replacement.

Regards.

Thanks to Neil and Kuba for their help. All methods can satisfy my needs. After testing, although Kuba's method has the shortest execution time, it will change the order of the fields. So I decided to use the second method provided by Neil! Thanks again for the help of Neil and Kuba ~~

n = 10;
assoc = Flatten[#] & /@ 
   Transpose[{Range[n], 
     Transpose[{Table[{2019, 10}, {i, 1, n}], 
       RandomChoice[Range[31], n], Table[{0, 0, 0}, {i, 1, n}]}], 
     RandomChoice[Range[100], n]}];
assoc = AssociationThread[{"id", "y", "m", "d", "h", "m", "s", 
       "Value"} -> #] & /@ assoc;
assoc[[All, "y"]] = 
  DateString@DateList[#] & /@ 
   Values@Query[All, {"y", "m", "d", "h", "m", "s"}]@assoc;
assoc = KeyDrop[assoc, {"m", "d", "h", "m", "s"}];
(*The second method provided by Neil*)
assoc = Map[KeyMap[ReplaceAll["y" -> "Date"], #] &, assoc];