To edit an absence, go to your Absences page and find the absence you wish to edit. You can do this in both Calendar and List view. In calendar view, click on the absence squares and then 'Edit absence' window will open.
In list view, find the absence in the list and click on the little pencil icon on the right hand side of it to open 'Edit absence' dialog.
To delete an absence, find the absence you wish to delete and click on it to open 'Edit absence' dialog and click on 'Delete'.