Added a new field to user profile, how do I set a value for existing users?
Use Views Bulk Operations Module to update field values


  This module augments Views by allowing bulk operations to be executed
  on the displayed rows. It does so by showing a checkbox in front of
  each node, and adding a select box containing operations that can be
  applied.



Create a view of type users 
Add Bulk operations: User field and choose Modify entity values checkbox to select your boolean field