[{"timeslot":[{time:"00:00",staff:["A"]}]}]

db.collection.update({},[{"$addFields":{"timeslot":{"$map":{"input":"$timeslot","as":"ts","in":{"$cond":{/** criteria for element you want to update*/"if":{$eq:["$$ts.time","00:00"]},"then":{"$mergeObjects":["$$ts",/** to-be version of staff*/{staff:["A","B"]}]},"else":"$$ts"}}}}}}],{multi:false})