[{"_id":"1234","arr":[1,2,3,4,5,6,7,8,9]}]

db.collection.update({_id:1234},[{$addFields:{gg:{$filter:{input:"$arr",as:"z",cond:{$gte:[{$indexOfArray:["$arr","$$z"]},5]}/** 5 is n first item*/}}},},{$project:{arr:"$gg",_id:1234,},},])