[{_id:"id",name:"name",loans:[[{id:1,acName:"ABC"}],[{id:2,acName:"DEF"}],[{id:3,acName:"GHI"}]]}]

db.collection.update({"loans":{$elemMatch:{$elemMatch:{id:2}}}},{$set:{"loans.$.$[loan].acName":"NEW_NAME"}},{arrayFilters:[{"loan.id":2}]})