Having trouble retrieving a users password

Hello everyone,
 
Could someone please help me with this problem I am having? I have an asp.net page that is only accesible to users with admin role, it is the adminsetup.aspx page. I want the admin to be able to change a user's password right there on the page. But I am having trouble doing so. I will show you my code.
 
This is the first thing I tried:
Protected Sub changeUserPassword() If String.IsNullOrEmpty(enterNewPasswordTextbox.Text) Then MsgBox("Please enter a new password") Return End If Dim userNameEntered As String = enterUserNameTextbox.Text Dim newPassWordEntered As String = enterNewPasswordTextbox.Text Try Dim theUser As MembershipUser = Membership.GetUser(userNameEntered) Dim usersOldPassword As String = theUser.GetPassword() theUser.ChangePassword ...