[{"_id":1,"Users":[{"name":"John Davies","age":null,"place_of_birth":"Cardigan"},{"name":"Edward Jones","age":null,"place_of_birth":null},{"name":"Daniel Rhys","age":NumberLong(63),"place_of_birth":"Cardigan"},{"name":null,"age":NumberLong(61),"place_of_birth":"Cardigan"},{"name":"John Davies ","age":null,"place_of_birth":"Cardigan"}]}]

db.collection.update({},{$set:{"Users.$[u].age":"missing"}},{arrayFilters:[{"u.age":null}],multi:true})