[{_id:"random123",days:[{_id:"1a",day:"Monday",tasks:["shopping","playing","movie"]},{_id:"1b",day:"Tuesday",tasks:["office","travel","relax"]},]}]

db.collection.update({_id:"random123"},{$set:{"days.$[d].tasks.$[t]":"enjoy"}},{arrayFilters:[{"d._id":"1b"},{"t":"office"}]})