This action will need to be done from the website at Expensify.com. You are not able to delete your account on the mobile app.
To delete your account go to the User Icon in the upper left hand corner of the screen > Account Settings > Account and scroll to the Delete Account section.
Please note: If your account is controlled by Domain Control, you cannot delete your own account. Only a domain admin can delete your account. Also, if you own any policies, you will need to delete these first as the deletion of your account will affect other policy users.
NOTE: deleting your account is not reversible!