Database
[{5:"cool",93:"cool",30:"cool",56:"cool",}]
Query
db.collection.update({},[{$replaceRoot:{newRoot:{$arrayToObject:{$filter:{input:{$objectToArray:"$$ROOT"},cond:{$cond:[{$regexMatch:{input:"$$this.k",regex:"^[0-9]+$"}},{$lt:[{$toInt:"$$this.k"},40]},true]}}}}}}])