A reservation can be modified in the app up to 1hr prior to your reservation check-in time. If you have already checked in, you will not be able to modify your reservation.
To do so, login to your Spot Club account and tap on the reservation you would like to edit on the home screen.
Once your reservation opens, tap Edit in the upper right hand corner and you will then be able to edit the location, check-in date/time, checkout date/time, parking type. If you made a Pay now reservation, you will be able to confirm or change the credit card used.