The best solution to this would be to have an “Options” page using ACF Pro (or the options page add on which is still available for ACF4 http://www.advancedcustomfields.com/add-ons/)
Then you can create an options page, add you custom field group to the options page and then pull the values from the options page for all of your pages.
Without using an options page you would need to use the acf/save_post hook http://www.advancedcustomfields.com/resources/acfsave_post/
You could do a query in you function to get all the pages and then update all of the fields on all of the pages.
