To modify a specific form, you use the FormsManager class and LINQ queries. The following code modifies a form with the specified newTitle through the Native API.
var formManager = FormsManager.GetManager();
var form = formManager.GetForms().Where(f => f.Id == formId).SingleOrDefault();
form.Title = newTitle;
var draft = formManager.EditForm(form.Id);
var master = formManager.Lifecycle.CheckOut(draft);
master = formManager.Lifecycle.CheckIn(master);
First, you initialize the FormsManager and assure that the form exists. Then, you must call GetForms to retrieve all forms and filter them based on the Id property. You assure that the form exists. You modify the title, edit and check out the form. Finally, you check in and publish the form draft and save the changes.