How to do Bulk updates of JIRA issues

If you ever wondered how to bulk change issues in JIRA, here's the procedure:

/!\ NOTE: when making a bulk edit (with or without the email notifications) it's usually a good idea to explicitly note in the comment that you are performing a bulk edit, and what criteria was used to find the issues so people who come along later and see the edit on a single issue will understand the larger context. It can be particularly helpful if you include some semi-distinct token (based on the date) to help identify all of the affected issues with a simple search, see for instance this query identifying issues with this comment.