Adding or removing a parent issue can be done on the "Edit issue" page.
To set or change a parent issue, enter the issue key of the parent issue in the "Parent issue" field.
(The parent issue must be within the same Project.)
To remove the parent issue, delete the issue key in the "Parent issue" field.
Press the "Submit" button after changing the setting.
When you change the parent issue's status to "closed", a dialog asking you whether it's okay to close its child issues will pop up.
When you press "Close all issues", the status of all child issues will be changed to "Closed".