Drupal-7 Where can I change the text from Simplenews unsubscription confirm page?
Sounds as if you need the String Overrides module. From the project page:


  Provides a quick and easy way to replace any text on the site.


This includes messages as well. Here's what you would need to do:


Extract to your sites/all/modules directory.
Enable the module in admin/build/modules.
Configure the access permission in admin/user/permissions.
Visit admin/config/regional/stringoverrides and fill in the strings you'd like to replace. This is the string coming from Simplenews: %user was unsubscribed from the %newsletter mailing list.. Just change it to whatever you would like. You have the %user and %newsletter replacements available.