Database
[{_id:"random123",days:[{_id:"1a",day:"Monday",tasks:["shopping","playing","movie"]},{_id:"1b",day:"Tuesday",tasks:["office","travel","relax"]},]}]
Query
db.collection.update({"days._id":"1b","days.tasks":"office"},{"$set":{"days.$[item].tasks.$[element]":"enjoy"}},{arrayFilters:[{"item._id":"1b"},{"element":"office"}],multi:true})