The OTA updates contain fixes and improvements to the functioning and performance of the vehicle, so right now, there’s no option to cancel/skip an update.
On the update screen, there is an option to schedule the installation at a later time. You can do this by tapping on the 3:00 AM button, which will give you options to schedule the installation. By default, the time of install is set at 3:00AM, since most people charge their vehicles at night.