[{_id:"xx",statuses:[{status:"pending",timestamp:"1 january 10 pm"},{status:"accepted",timestamp:"2 january 2 am"}]}]

db.collection.update({last_accepted_at:{$exists:false},"statuses.status":"accepted"},[{$set:{last_accepted_at:{$arrayElemAt:[{$filter:{input:"$statuses",cond:{$eq:["$$this.status","accepted"]}}},0]}}},{$set:{last_accepted_at:"$last_accepted_at.timestamp"}}])