You can't edit but have to delete the leave and then request a fresh leave. 

To delete the leave:

1) Please type "delete" as a direct message to the app and you will a list of all the leaves applied. 

2) Select the leave you wish to delete and click on "delete" button. 

3) You can then apply for a new leave by typing "Apply" to AttendanceBot and selecting the relevant details mentioned in this article