How does mongodb implement updating the value of one field to the value of another field?

  mongodb, question 
db.CargoUserProfiles.find().forEach(
 function(item){
 db.CargoUserProfiles.update({"id":item._id},{"$set":{"LastUpdate":item.CreateAt}},false,true)
 }
 )
 
 db.CargoUserProfiles.find({}).forEach(function(asset){
 asset.LastUpdate = asset.CreateAt;
 })

The above two methods were tried but failed. CreateAt is a field with data and LastUpdate is a field to add

db.CargoUserProfiles.find().forEach(
 function(item){
 db.CargoUserProfiles.update({"_id":item._id},{"$set":                    {"LastUpdate":item.CreateAt}},false,true)
 }
 )
 
 The id field in mongodb is "_id" instead of "id"

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