A value in an object can be updated by assignment. If the property name already exists in the object, the property value is replaced:

stooge['first-name'] = 'Jerome';

If the object does not already have that property name, the object is augmented:

stooge['middle-name'] = 'Lester';
stooge.nickname = 'Curly';
flight.equipment = {
    model: 'Boeing 777'
};
flight.status = 'overdue';