The employees can edit/modify the submitted expense only if it's not approved by the manager (in pending approval status)
Just type "Modify" as a direct message to the app and select the expense you wish to edit and click on "Edit details".
You can then edit a bunch of things like amount, purpose, date, receipts and click on "Finish Editing"
If an expense has already been approved by the manager, then the only way to modify it is for the manager to say "Modify @username" and click on edit details.
Note: An approved expense cannot be modified by the employee.