[{"_id":1,"grades":[{"grade":80,"mean":75,"std":6},{"grade":85,"mean":100,"std":4},{"grade":85,"mean":100,"std":6}]},{"_id":2,"grades":[{"grade":90,"mean":100,"std":6},{"grade":87,"mean":100,"std":3},{"grade":85,"mean":100,"std":4}]}]

db.collection.update({},{"$inc":{"grades.$[elem].std":-1}},{arrayFilters:[{"elem.grade":{"$gte":80},"elem.std":{"$lte":5}}],multi:true})