Change/modify the password of FBA user using the sqlmembershipprovider

I have implemented FBA in SP 2010 on my dev server with sp 2010 sp2 and sql serevr2008 r2 sp1.

configured the users using the aspnetdb and sqlmembership provider.Now i want to change the password for the users  whoare members of the site/subsites.

how to achieve this?

i tried with the below code in a testing asp.net 3.5 fwrk web appln, but it didnt work:

namespace TestingWebApplication1
{
    public partial class _Default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {

            if (!Page.IsPostBack)
            {
                //string username = "username";
                //string password = "newpassword";
                //MembershipUser mu = Membership.GetUser(username);
                //mu.ChangePassword(mu.ResetPassword(), password);


                string username = "opuser1";
                string password = "pass@word";

               MembershipProvider mpp = Membership.Providers["OPMembership"];
               MembershipUser mmuser = mpp.GetUser(username, false);

              mmuser.ChangePassword(mmuser.ResetPassword(), password);
            }

        }
    }
}

throws me the object ref. Error