Events and classes can be bulk edited to assign categories, languages, related festival years, and other taxonomies, or to publish or unpublish many events quickly.
You cannot use this feature to un-assign categories, so be careful what you add! Un-assigning has to be done one Event at a time.
Navigate to Events >> Events.
Select all the events you want to edit using the left-hand checkboxes.
Near the top of the page, select Bulk Actions >> Edit, then click "Apply"
Select the Series, Genre, Languages, Categories, Festivals, and status changes that you want to apply to the group
Press the blue "Update" button to apply the changes to all selected events.
Also note the "Quick Edit" interface visible on this screen to make event-by-event changes quickly without opening the full event.