You must have a supervisor, manager or admin role in order to have access to this feature on the mobile app.
The procedure goes as follow:
Go on the mobile app
Choose the "Request" menu
Press on "Time-offs"
Choose the time-off request to manage
Select the time-off settings (type, etc.)
That's all !
Warning: The app will warn you if the time-off request is in conflict with an existing shift. However, for the moment it is not possible to reassign the shift to another employee from the mobile app. You will have to go on the web-app to do so.