Module to edit field data on multiple instances/nodes (in bulk)
For Drupal 7 you will need views bulk operations which packages this and other bulk operations.  For Drupal 8 you need Bulk Update Fields 

Below the procedure is for Drupal 8 but very similar for Drupal 7

Install the module and modify the Admin content view found by going to admin/structure/views/view/content or perhaps you can use any view that you want.

add field and search for Node operations bulk form to add and make sure you enable 

Selected actions: Bulk Update Fields to Another Value

With this when on the view page you can select 1 or more nodes and modify fields in bulk