Update Doesn't Work In ADO.net Entity Data Model?

I use ADO.net Entity Data model for work with database.

In my program, I want to update a record of user table, so I use the code below to do this.

In this function I send changed user info and then overwrite the information with the current user information.

After I run objUser = _user; and then call objContext.SaveChanges(); to save the changes.

But when I do this, the changes are not persisted to the database. I use this code for another programs but in this case the code does not work!

public void Update(tbLiUser _user)
{
LinkContext objContext = this.Context;
tbLiUser objUser = objContext.tbLiUsers.First(u => u.tluId == _user.tluId);
objContext.Attach(objUser);
objUser = _user;
objContext.SaveChanges();
}