[{cards:{starter:[],intermediate:[],advanced:[{cards:[{title:"this is a card",id:"main-2-1"},{title:"this is a card",id:"main-3-1"},{title:"this is a card",id:"main-4-1"},],unit:2}]},}]

db.collection.update({"cards.advanced.unit":2},[{$set:{"cards.advanced":{$map:{input:"$cards.advanced",as:"adv",in:{cards:{$map:{input:"$$adv.cards",as:"advcard",in:{$cond:[{$eq:["$$advcard.id","main-2-1"]},{title:"this is a NEW updated card",id:"$$advcard.id"},"$$advcard"]}}},unit:"$$adv.unit"}}}}}],{new:true,})