[{"_id":1,"username":"bob","pause":true,"pause_date":ISODate("2021-07-16T07:13:48.680Z"),"learnt_item":[{"memorized":false,"character":"一","next_review":ISODate("2021-07-20T11:02:44.979Z")},{"memorized":false,"character":"二","next_review":ISODate("2021-07-20T11:02:44.979Z")}]}]

db.collection.update({"_id":1},[{$set:{"pause_date":null,"pause":false,"learnt_item":{$map:{input:"$learnt_item",as:"item",in:{$cond:[{$eq:["$$item.memorized",false]},{memorized:"$$item.memorized",character:"$$item.character",next_review:{$add:["$$item.next_review",345600000]}},"$$item"]}}},}}],{new:true,})