[{user_id:"13",stock:[{part_id:"P456",quantity:3},{part_id:"P905",quantity:8}]}]

db.collection.update({user_id:"13"},[{$set:{stock:{$cond:[{$in:["P456","$stock.part_id"]},{$map:{input:"$stock",in:{$cond:[{$eq:["$$this.part_id","P456"]},{part_id:"$$this.part_id",quantity:{$add:["$$this.quantity",1]}},"$$this"]}}},{$concatArrays:["$stock",[{part_id:"P456",quantity:1}]]}]}}}])