MongoDB – Update or Insert object in array

Try this

db.collection.update(
    { _id : ObjectId("57315ba4846dd82425ca2408")},
    { $pull: {"myarray.userId": ObjectId("570ca5e48dbe673802c2d035")}}
)
db.collection.update(
    { _id : ObjectId("57315ba4846dd82425ca2408")},
    { $push: {"myarray": {
        userId:ObjectId("570ca5e48dbe673802c2d035"),
        point: 10
    }}
)

Explination:
in the first statment $pull removes the element with userId= ObjectId("570ca5e48dbe673802c2d035") from the array on the document where _id = ObjectId("57315ba4846dd82425ca2408")

In the second one $push inserts
this object { userId:ObjectId("570ca5e48dbe673802c2d035"), point: 10 } in the same array.