will software update 13.3.1 increase my storage or decrease it

Installing the update might slightly alter the available storage capacity of your device - it will not change the overall storage on the device.

Unless you are already running out of storage space on your device (additional temporary storage is needed for the actual install process) the difference will be trivial.

It will slightly decrease it.

iOS 12 and iPadOS take up about the same amount of storage space.

iPadOS will take up slightly more.