[{_id:ObjectId("619365c54a7d53716438933e"),name:"Chris",hobbies:[{title:"Sports",frequency:5},{title:"Cooking",fequency:5},{title:"Hiking",frequency:1}],isSporty:true,cookMaster:true,age:35}]

db.collection.update({name:"Chris"},[{$set:{"hobbies":{$map:{input:"$hobbies",in:{$mergeObjects:["$$this",{frequency:"$$this.fequency"}]}}}}},{$unset:"hobbies.fequency"}])