From this option of the Bulk Operations, you'll be able to associate, change or remove the category from one or many projects.
1. First of all, you need to filter through the projects you want to assign/change the category to.
2. Select Bulk operations > Category.
3. On this screen, choose the category you want to associate to the projects you just selected.
4. After selecting the category and clicking on 'Next', you'll see the confirmation screen, where you'll need to confirm the actions you are about to make. Please, read carefully.
5. By selecting the 'Confirm' button, the operation will start and. As soon as it's done you'll be able to see the changes that have been made.
This is all you need to do in order to change/associate a category to one or many projects.