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If you want to change password of currently logged in user. Then following code can be helpful for you.
 
Boolean changepass = UserController.ChangePassword(this.UserInfo,
  YourOldPassword.Text, YourNewPassowrd.Text);
 
if (changepass)
{
    DotNetNuke.UI.Skins.Skin.AddModuleMessage
         (this, "Your Password has been changed successfully",
         DotNetNuke.UI.Skins.Controls.ModuleMessage.ModuleMessageType.GreenSuccess);
}