How to update Local Storage array

If we have an array of objects in local storage and we want to change a specific field of an object at a given index how can we do this. In my case I have an array named “bookings” and in this array there are multiple json objects, I want to change the some fields of json object like starttime of booking which is at index=pointer, pointer is a variable pointing the booking on which we want to make changes. I have tried this one local1.data.bookings[local1.data.pointer].set(‘starttime’,var1.datetime) but it doesn’t worked.