Issue tickets can be deleted as well, given you have permissions to create issues:
- Open the issue you want to extend the deadline for by clicking the icon from one of the issue listings.
- Gived you have permissions to create and edit issues, click the Edit button in the lower right corner of the window.
- Click the Delete button in the lower left corner of the window, and confirm.
Be aware that by deleting an issue ticket, all its content, including comments and changelog will be deleted.