Database
[{"_id":ObjectId("57315ba4846dd82425ca2408"),"myarray":[{userId:ObjectId("570ca5e48dbe673802c2d035"),point:5},{userId:ObjectId("613ca5e48dbe673802c2d521"),point:2}]}]
Query
db.collection.update({_id:ObjectId("57315ba4846dd82425ca2408")},[{$set:{myarray:{$cond:[{$in:[ObjectId("570ca5e48dbe673802c2d035"),"$myarray.userId"]},{$map:{input:"$myarray",in:{$mergeObjects:["$$this",{$cond:[{$eq:["$$this.userId",ObjectId("570ca5e48dbe673802c2d035")]},{point:3},{}]}]}}},{$concatArrays:["$myarray",[{userId:ObjectId("570ca5e48dbe673802c2d035"),point:4}]]}]}}}])