Change password for user!

I use aspnetdm.mdf for. I use CreateUserWizard to create user. I want change password for users. I use dropdownlist with username and textbox(for write new password) and button. When i click button i want change password for selected user from dropdownlist on password password from textbox. First i use
Membership.Provider.ChangePassword(this.DropDownList2.SelectedItem.Text, p1, p2);

p1 - old password
p2 - new password but i dont now old password

Then i find 
aspnet_Membership_SetPassword
But i don't understand how it's work. Can i execute it from my web app.
onnection = new SqlConnection(connectionString); string commandText = "aspnet_Membership_Se ...