Update/Change a Password using Membership.

Hey Folks,
I can't figure out how to change an existing user's password.  I'm using membership in VS10 .NET 4.0.  I want to update an existing user's password if the user exists, and I cannot figure out how to update the password.
string sUser = TextBoxUserName.Text.Trim();
string sPwd = TextBoxPwd.Text.Trim();
MembershipCreateStatus status;
try { // add if the user exists
Membership.CreateUser(sUser, sPwd, ... , out status);
if( status == MembershipCreateStatus.DuplicateUserName )
{ // Update the password ??? }
}
catch ( Exception eX) {
LabelError.Text = eX.Message; }
I just can't find the API to update teh password - like Membership.UpdatePassword("Dave","NewWorld");
Thanks ...
...