Database
[{"Orders":[{"CustomFields":{"Weight":{"FieldId":ObjectId("6149d5de44175b8324482915"),"Type":"Number","ValueBson":2222}},"Number":"21312"}]},{"Orders":[{"CustomFields":{"Weight":{"FieldId":ObjectId("6149d5de44175b8324482915"),"Type":"Number","ValueBson":2222}},"Number":"21312"}]}]
Query
db.collection.update({},[{$set:{Orders:{$map:{input:"$Orders",in:{$mergeObjects:[{"Number-test":"$$this.Number"},"$$this"]}}}}},{$unset:"Orders.Number"}],{multi:true})