Database

[{"key":1,"numbers":[0,0,0,{"04/03/2023":[0,5,0]}]},{"key":2,"numbers":[0,0,0,{}]}]

Query

db.collection.update({key:1},[{$addFields:{"numbers.3.04/03/2023":{$cond:{if:{$isArray:"numbers.3.04/03/2023"},then:{$concatArrays:[{$slice:["numbers.3.04/03/2023",0,1]},[{$add:[{$arrayElemAt:["numbers.3.04/03/2023",1]},1]}],{$slice:["numbers.3.04/03/2023",2,1]}]},else:[0,1,0]}},}}])

Result