How to update only one field using Entity Framework?

Here's the table

Users

UserId
UserName
Password
EmailAddress

and the code..

public void ChangePassword(int userId, string password){
//code to update the password..
}

Update: If you're using EF Core 7.0 or above, see this answer.

Ladislav's answer updated to use DbContext (introduced in EF 4.1):

public void ChangePassword(int userId, string password)
{
    var user = new User() { Id = userId, Password = password };
    using (var db = new MyEfContextName())
    {
        db.Users.Attach(user);
        db.Entry(user).Property(x => x.Password).IsModified = true;
        db.SaveChanges();
    }
}