using CMS.EventManager;

int eventNodeId = 10;

string email = "john.smith@example.com";

// Get attendee info object

EventAttendeeInfo eai = EventAttendeeInfoProvider.GetEventAttendeeInfo(eventNodeId, email);

// Update its properties

eai.AttendeeFirstName += "_Updated";

// Save the object

EventAttendeeInfoProvider.SetEventAttendeeInfo(eai);