Handles are individual "tags" that allow members to tag other members within a Request. This allows for collaboration with other organizational members during the course of a Request's life cycle. When you "tag" someone using their handle in an internal comment, the tagged user will be notified by email.
Can Handles be changed after they are set?
An owner cannot update a handle, only the SeeClickFix (now Citizen Relationship Management) administration has the capability. You can reach out to Support to request a handle change.