Database
[{_id:1,events:[{event:1,timestamp:"1619463804.069353"},{event:2,timestamp:"1649463804.069353"}]}]
Query
db.collection.aggregate([{$addFields:{events:{$map:{input:"$events",as:"event",in:{"$mergeObjects":["$$event",{timestamp:{"$toDate":{$multiply:[{"$toDecimal":"$$event.timestamp",},1000]}}}]}}}}}])