The available actions for a duty depend on it's state. In most cases it should be clear what is available. Remember that you must be a Duty Admin.
See Getting Started with Duties.
Use the "Reassign" button and select the new assignee. Notifications will be sent to both the old and new assignees. If the new assignee is left empty, then the duty goes back into the "Unassigned" pool.
This can be done retrospectively, i.e. you adjust old records if needed.
Use the "Delete" button. The duty is removed. A notification is sent the current assignee, if there was one.
See Adding Duties.
See Duty Reports.
There may be rules attached to a duty type that stop a user from volunteering. They will see an error message that includes "Think this should be allowed? You can ask a Duty Admin to make this change."
You can override these rules. Use the "Assign" button and pick the user. On the first attempt the assignment will fail for you as well. Tick the "Ignore Failures" checkbox and assign again.