i guess you redirect to the Home.aspx if the login details matches. so my suggestion is to do changes from page load on home.apsx.put following code.
protected void Page_Load(object sender, EventArgs e)
((Label)Master.FindControl("signup2")).Text = Session["Username"].ToString();
using that code you can access master pages's controls. But my suggestion is to have separate links to Login Log out and register. you can show or hide them as it needed.
thanks. hope this helps.