[{"_id":"123","firstArray":[{"_id":"456","status":"open","nestedArray":[{"_id":"100","quantity":10},{"_id":"101","quantity":10},{"_id":"102","quantity":10}]},{"_id":"789","status":"open","nestedArray":[{"_id":"200","quantity":10},{"_id":"201","quantity":10},{"_id":"202","quantity":10}]}]}]

db.collection.update({"firstArray.nestedArray._id":"101"},[{$set:{firstArray:{$map:{input:"$firstArray",in:{$mergeObjects:["$$this",{nestedArray:{$map:{input:{$filter:{input:"$$this.nestedArray",cond:{$ne:["$$this._id","201"]}}},in:{_id:"$$this._id",quantity:{$cond:[{$eq:["$$this._id","101"]},20,"$$this.quantity"]}}}}}]}}}}}])