Parking standards supplied by Autodesk can only be edited once they have been selected for a project.
Then, any changes you make to the standard will affect not only new bays but also any bays that have already been placed. Once you have used a standard, all bays placed will use that standard.
You may edit parking standards that you create. However, in order to prevent accidental changes, they will be locked when you close Autodesk Vehicle Tracking and you need to unlock them to make any changes.