var ViewModel = function () {
    self.programs = ko.observableArray([
        {programId: 1, programDescription: 'One'},
        {programId: 2, programDescription: 'Two'},
        {programId: 3, programDescription: 'Three'}
    ]);
    self.program = ko.observable();

    self.saveProgram = function () {

        for (i = 0; i < self.programs().length - 1 ; i++) {
            if (self.programs()[i].programId == self.program().programId) {
                self.programs()[i].programDescription =
                    self.program().programDescription;
                alert(self.programs()[i].programDescription);
            }
        }

    };

};

ko.applyBindings(new ViewModel());

<div>
    <select data-bind="options: programs,
                       optionsText: 'programDescription',
                       value: program"></select>
</div>
<div>
    Update Program Description: <input type="text" data-bind="value: program().programDescription" />
    <button type="button" data-bind="click: saveProgram">Update</button>
</div>